How did "sand" come to mean courage/pluck?

Question

How come sand means courage/pluck?

There isn't much information available on the Internet regarding its etymology.

With word etymologies I think the buck stops with the redoubtable World Wide Words, but in this case it doesn't sound thoroughly convincing:

Sand here has just the same sense as the older grit, clear grit, or true grit, that refer to a person who has strength of character, pluck, stamina, the ability to see things through to the end. The reference here, presumably is to the toughness of grit, especially that in gritstone, a common name for the material that made up the stones of a corn mill.

Why sand should suddenly pop up in its place — sometime near the end of the 1860s — is hard to say, though it is an obvious enough synonym.

Mark Twain uses both grit and sand in successive sentences in that place in Huckleberry Finn: “She had the grit to pray for Judus if she took the notion — there warn’t no backdown to her, I judge. You may say what you want to, but in my opinion she had more sand in her than any girl I ever see; in my opinion she was just full of sand”. How these became interchangeable terms is a small mystery that needs to be resolved, but don’t bank on anyone finding the solution anytime soon!

[World Wide Words]

Anyone in the know of the roots of sand as used in this sense?

Answer 1

Why do you find World Wide Words’ explanation (“it is an obvious enough synonym”) unconvincing?

Oxford English Dictionary (login required) indicates that sand is used in the sense of the earlier grit (meaning pluck, stamina). (See the note at sand: Cf. GRIT n.¹ 5.) . . .

sand, n.²
7. slang.
b. Chiefly U.S. Firmness of purpose; pluck, stamina. sand in one’s craw. Cf. GRIT n.¹ 5.
_{[selected examples, including earliest noted]
1867   G. W. HARRIS Sut Lovingood 102   I tell yu he hes lots ove san’ in his gizzard; he is the best pluck I ever seed.
1872   Newton Kansan 5 Dec. 3/3   We hope to see Mr. Pettibone with sufficient ‘sand in his craw’ for this new position [sc. police judge].
. . .
1881   N.Y. Times 18 Dec. in Notes & Queries 6th Ser. 5 65/2   Sand. To have ‘sand in one’s craw’; to be determined and plucky. Equivalent to ‘grit’.
. . .
1884   ‘M. TWAIN’ Adventures Huckleberry Finn viii. 65   When I got to camp I warn’t feeling very brash, there warn’t much sand in my craw.
1924   J. GALSWORTHY Forest iv. ii. 120   By Jove, Mr. Farrell, there’s sand in you. Tell me, isn’t he ever ashamed of himself?}

Here’s the cf.:

grit, n.¹
5. colloquial.
a. Originally U.S. slang. Firmness or solidity of character; indomitable spirit or pluck; stamina. to be clear grit, hard (etc.) grit: to have genuine spirit or pluck. to be the grit: to be the ‘right sort’, the genuine ‘article’.
_{[selected examples, including earliest noted]
1825   J. NEAL Brother Jonathan III. 386   Proper fellow he was too; ‘cute enough, I tell you—sharp as a razor—clear grit.
. . .
1863   N. HAWTHORNE Our Old Home II. 179   His main deficiency was a lack of grit.
. . .
1892   Times 23 Apr. 7/1   Every appointment of the kind must be based wholly upon fitness and grit.}

Answer 2

Sand has been slang for courage since the mid-1800s. Here are a few instances:

1847 [US] J.S. Robb Streaks of Squatter Life 73: He set his brain to work conning a most powerful speech, one that would knock the sand from under Hoss.

c.1858 [US] J.H. Green Reformed Gambler 121: I tell you, I never had the sand so knocked from under me before in my life. If you preach in that way, there wont be many of us gamblers left on this boat.

But with the same sense grit has an older usage, from which the figurative meaning of sand may probably derive:

Grit:

(US) solidity or strength of character, spirit, pluck, stamina; thus be the grit v., to be the ‘right sort’ of person, to be the ‘genuine article’.

1808 D. Hitchcock Poet. Dict. 53: The prude doats on beauty, the bully on grit [DA].

1809 [US] A.B. Lindsley Yankee Notions 10: ’E’s proper stuff; clear grit, I swow!

1825 [US] J. Neal Brother Jonathan II 14: A chap who was clear grit for a tussle, any time.

(Green’s Dictionary of Slang)