Dictionary discussions of 'hard as nails' and 'tough as nails'
Christine Ammer, The American Heritage Dictionary of Idioms, second edition (2013) has an entry for "hard as nails" but not for "tough as nails":
hard as nails Unyielding, callous, unsympathetic, as in Don*'t ask her for a contribution—she's hard as nails*. This term has replaced the 14th-century simile hard as flint stone and presumably alludes to the rigidity of nails.
Ammer offers a somewhat different entry for "hard as nails" in Ammer, The Facts on File Dictionary of Clichés, second edition (2006):
hard as nails Unyielding, tough, usually describing a person. This simile, which replaced the earlier hard as flint or stone (dating from Chaucer's time), seems to allude to a nail's ability to withstand the blows of a hammer. Shaw uses it in at least three of his plays (You Never Can Tell, Heartbreak House, Man and Superman) to describe an unsentimental character.
Cambridge Dictionary of American Idioms (2003) takes the view that "tough as nails" is simply a variant of "hard as nails":
(as) tough as nails, also (as) hard as nails strong and determined | She is a warm and friendly person, but she is also as tough as nails.
Clearly there is some difference in meaning between Ammer's "hard as nails" and Cambridge's "tough [or hard] as nails." Adam Makkai, A Dictionary of American Idioms, revised edition (1975) suggests that "hard as nails" can mean either of two distinct things:
hard as nails adj. phr., informal 1 Not flabby or soft; physically very fit; tough and strong.—A cliché. After a summer of work in the country, Jack was as hard as nails, without a pound of extra weight. (Jack was strong and tough.) 2 Not gentle or mild; rough; stern.—A cliché. Johnny works for a boss who is hard as nails and scolds Johnny roughly whenever he does something wrong. (His boss is very rough and strict.)
It is possible that "tough as nails" applies only to situations involving Makkai's definition 1, while "hard as nails" can apply to situations involving either definition 1 or definition 2. Alternatively, "tough as nails" may apply to situation involving Makkai's definition 1 and to situations involving a third meaning, along the lines of "extremely resilient or damage resistant"—where "hard as nails" does not apply or applies only rarely.
As Ammer points out in her Dictionary of Clichés, "hard as nails" usually arises as a characterization of a person. The same appears to be true of "tough as nails."
In any case, if "tough as nails" arose as a variant form of "hard as nails" which in turn arose as a variant form of "hard as flint" or "hard as stone," it has a very long pedigree indeed.
Early instances of 'tough as nails'
The earliest Google Books match for "tough as nails" is from "Some More Chapters in the History of John Bull," in Punch, or the London Charivari (February 28, 1857):
Well, my brace of Scotchmen went up to the farm, and like shrewd hard-working men of business, as they were, at once set about the inquiry Pam had charged them with/ At first the keepers tried their grand airs on the pair—were snappish, and saucy, and humorous, and mighty short in their answers, with "marry, come ups." and "you ask me no questions, and I'll tell you no lies," and "don't you wish you may get it," and so forth. And, truly, if the two old fellows had not been as tough as nails, and as patient as a brace of Jobs, they might have lost temper a thousand times, and most likely flung up their task in disgust.
The next earliest is from Walter Thornbury, "In Africa" in the Household Words (March 12, 1859):
It was a throb and struggle of oars, that spread out now like swallow's wings, now like the legs of a centipede—a pull, a sway, a lug at a rope, and we were on board the zebec, where we soon, Fluker and I, took up our quarters, near the immensely long handle of the tiller, which, in true lazy Spanish fashion, was managed by a rope, held by a fat, bare-footed sailor, who steered sitting down ; which did not startle me, because I remembered that the helmsman of the Seville steamer, though a rogue "tough as nails," had a sort of music stool, to enable him get through his laborious work.
Thornbury, an English author, published a complete version of his travels, including this account, in Life in Spain: Past and Present (1860).
And from "Old Murder" in All Year Round (the successor to Household Words) (May 5, 1866):
"Let it be the last time, Jack," said the doctor; "it is harder to come up hill one step, than to go down twenty. Do not break my heart by becoming a bad man. By-the-by, have you sent Aunt Fanny the medicine, and how is she?"
"Oh, pulling through all right. She's as tough as nails."
The earliest unique match for "tough as nails" in the Elephind newspaper database, meanwhile, comes from "Local Intelligence," in the [Sydney, New South Wales] Bell's Life in Sydney and Sporting Chronicle (November 2, 1861):
Our unfortunate Ministers have to face an opposition just now,and no mistake ; but never mind, they know what they are about. They regard it as mere fun. "Censures" will never break their hearts. They are as tough as nails, and defy the puny efforts of their once bosom "friends." The Cowper motto is "No Surrender!"
The first unique occurrence of "tough as nails" in a U.S. publication in the Elephind database doesn't appear until 1892.
It seems then that "tough as nails" probably emerged in Britain, with much the same meaning as "hard as nails" in the sense of "tough, resilient, not easily overcome or defeated" by 1857, and reached Australia by 1861, but did not become locally popular in the United States until the early 1890s.
Early instances of 'hard as nails'
Matches for "hard as nails," meanwhile, go back to at least 1820. From John Clare, "My Mary," in Poems Descriptive of Rural Life and Scenery (1820):
Who, frost and snow, as hard as nails, / Stands out o' doors, and never fails / To wash up things and scour pails? / My Mary.
And from "Notes on the United States of America," in Blackwood's Edinburgh Magazine (November 1828):
The honours to which General Jackson is aspiring demand the extracting of the following notice of him:
"Mohawk country. ...He [Andrew Jackson] is considered by all, even by his friends, to be a rough back-woodsman, Indian kind of soldier ; and his credit was gained by his success over our [British] troops at New Orleans, and his destroying of the hostile Indians. 'As bold as a lion, as tough as hickory, as hard as nails, but as unlicked as a bear's cub.' His military reputation may, in the event of his being elected President, be productive of serious events ; should he prove ignorant and incapable in his civil capacity, he may wish to hide his deficiencies by playing at soldiers. ..."
Although the source of this "notice" goes unidentified, the writer is clearly British as he refers to the defeat of "our troops" at the Battle of New Orleans. The quotation within the quotation, however, appears to be from a U.S. source, and it is there that "hard as nails" appears. If that writer (or speaker) is indeed from the United States, "hard as nails" was in use in that country at least 64 years before the earliest native U.S. instance of "tough as nails" that I have been able to find.
Early instances of 'hard as stone' and 'hard as flint'
The antecedents to "hard as nails," according to Christine Ammer, are "hard stone" and "hard as flint. A search of Early English Books Online yields literal instances of "hard as stone" from as early as Thomas Elyot, Bibliotheca Eliotæ Eliotis Librarie (1542):
Ebenus, uel Ebenum, a tree whiche is black in colour, and is odoriferous, whā it is burned it hath vertue to clense the eyen. This tree groweth in India, whiche beinge cut, waxeth as hard as stone. Sola India nigrum fert ebenū: India alone bryngeth blacke ebenns. it is also written with, h. rede Hebenus.
and figurative use of "hard as stone" from as early as a 1554 translation of Giovanni Baccaccio, The Tragedies, Gathered by Ihon Bochas, of All Such Princes as Fell from Theyr Estates throughe the Mutability of Fortune since the Creacion of Adam ...:
The night came on darked with ignoraunce, / My wit was dull by clerenes to discerne / In Rethoryke for lacke of suffisaunce, / The torches out & quenched was the lantern, / And in this case my style to gouerne / Me to further I founde none other muse, / But hard as stone Pierides and Meduse.
And from a 1583 translation of John Calvin, The Sermons of M. Iohn Caluin vpon the Fifth Booke of Moses Called Deuteronomie ...:
Therefore it behooued the lawe to be renewed, and that God shoulde publishe it after an other fashion, not writing it in tables of stone, but in our heartes by chaunging them. For by nature our harts are as hard as stone, as it is said by the Prophet Ezechiel. Therefore must God be faine to soften them, & to make thē plyable, yt they may be obedient to his lawe.
For its part, "hard as flint" appears figuratively at least as early as a 1568 translation of Antonio de Guevara, The Dial of Princes:
I ly, if I saw not once in the county of Aragone, a gentilmā that hyred a whole howse, wheare him selfe and his famely were very well lodged and comodiously: & after that I remembred I met with him in Castilla, wheare hee could not content him selfe with the change of eight howses, beesides his first hee was appointed to: and the occasion was, for that in Aragon hee paied for that howse hee had, and for these hee paied nothing. So of others purse euery man coueteth to showe his magnificence, and to declare his follyes: but when they deffray their owne charge, they are hard as flynt, and goe as neere to woorke as may bee.
And from a 1570 translation of The Pityfull Histori[e] of Two Louing Italians, Gaulfrido and Barnardo le Vayne:
Such comely persons to behold / as they along did go, / Would make a man as hard as flinte / to melt, my selfe I know: / But for to thinke what piteous lookes / they shewed as they went, / Agayne to sée what smiling chéere / the one the other lent, / To sée what wonderous changes was / betwéene these troubled thrée, / Tho that I liude seuen hundred yeres / the like I should not see.
The earliest confirmed instances of "hard as nails" in Google Books search results are about thirty years older than the earliest confirmed instances of "tough as nails," which lends plausibility to the idea that "tough as nails" emerged as a variant of "hard as nails," rather than independently of it. "Hard as nails," in turn, appears long after the kindred terms "hard as stone" and "hard as flint." So one possible answer to the question of where "tough as nails" came from is that it may have come from "hard as stone [or flint]," which, according to Christine Ammer, go back to the 1300s.
A more narrowly focused question—when did "tough as nails" appear in that particular idiomatic form?—brings us closer to the present. The earliest match that I have been able to find dates to 1857. As in the case of "hard as nails" from three decades earlier, all of the earliest instances of "tough as nails" come from Great Britain.