Early 'scepticks' and 'skepticks'
The earliest occurrence of sceptical (or skeptical) that I've been able to find in an English dictionary is in Edward Phillips, The New World of English Words, or a Generall Dictionary (1658):
Sceptical, (Greek) contemplative, whence Scepticks are a sort of Philosophers who onely consider and contemplate of things without determining any thing.
John Kersey, The New World of Words: or, Universal English Dictionary (1706) has this series of relevant entries:
Sceptical or Sceptick, belonging to the Scepticks or Scepticism ; that is in doubt or suspence, doubtful.
Scepticism, the Doctrine or Opinion of the Scepticks.
Scepticks, a Sect of Philosophers, who contemplated and consider'd Matters, but doubted of every thing, and would admit of no Determination : Whence the Term is apply'd to a Person who maintains there is nothing certain, and no real Knowledge at all to be had.
To like effect is Kersey's successor, Nathan Bailey, An Universal Etymological English Dictionary, second edition (1724) [you have to scroll down to the sceptical entry, unfortunately, because Google Books refuses to find matches from the relevant page]:
SCEPTICAL, SCEPTICK {Sceptique, F. Scepticus, L. Σχεπτχις, Gr.} belonging to the Scepticks or Scepticism, Contemplative, that is in Doubt or Suspence, Doubtful.
SCEPTICISM, the Doctrine and Opinions of Scepticks.
A SCEPTICK, {Sceptique, F. Scepticus, L. of Σχεπτχις, Gr. of το Σχεπτεσϑ Gr. to look out or observe, to contemplate} a Sect of Philosophers who contemplated and considered Matter, but doubted of every Thing, and would admit of no determination, thence the Term is applied to those who maintain there is nothing certain and no real Knowledge at all to be had, but that a Man ought to doubt and disbelieve every thing.
Oddly enough, the first dictionary to use the spelling use of skeptical is Nathan Bailey, An Universal Etymological English Dictionary, "The Second Edition with many Additions" (1731), which provides full entries for scepticalness, scepticism, and scepticks, but also full entries for skeptic/skeptick, skeptically, and skepticism, the last defined as follows:
SKEPTICISM, the doctrine and opinions of the skepticks ; which was, that Persons ought to suspend their judgment, as to the determination or firm belief of any thing.
Samuel Johnson, who seems to have been no admirer of the ancient skeptics and who famously attempted to refute the more recent Bishop Berkeley school of doubt by kicking a stone, weighed in on the side of skeptical. Here are the relevant entries in Samuel Johnson, A Dictionary of the English Language, volume 2 (1756):
SCEPTICK. s. See SKEPTICK.
...
SKEPTICK. s. {ςκεπλομαι} One who doubts, or pretends to doubt of every thing. Decoy of Piety. Blackmore.
SKEPTICAL. a. {from skeptick.} Doubtful ; pretending to universal doubt. Bentley.
SKEPTICISM. s. Universal doubt ; pretence or profession of universal doubt. Dryden.
Johnson's preference was far from universal, however, as is evident in the latest effort from the ever-unpredictable Nathan Bailey, The New Universal Etymological English Dictionary, fourth edition (1756), which dumps all of the entries with k as the second letter in favor of entries for sceptical, scepticalness, and scepticism.
'Sceptics' and 'skeptics' in the New World
I was somewhat surprised to find that Noah Webster, A Compendious Dictionary of the English Language (1806) ignores the k spelling altogether:
Sceptic, n. one who doubts of every thing, especially of the truth of revelation
Sceptical, a. doubting, hesitating to admit the truth of an opinion or system
Sceptically, ad. in a sceptical manner
Scepticism, n. doubt, hesitation to admit the truth of revealed religion
Scepticize, v. t. to doubt, to be sceptical.
Likewise Webster's first full-size An American Dictionary of the English Language (1828) provides entries for the same five spellings as the Compendious Dictionary did, plus a brief cross reference:
SKEPTIC. {See Sceptic.}
Webster announces his break with the form sceptic in his July 1841 preface to An American Dictionary of the English Language, second edition (1846):
In a class of words which have borne two forms, the author selects that which he deems most proper, and discards the other. Thus he prefers to write afterward, backward, forward, onward, toward, &c, without the s. He rejects amongst and whilst, as obsolete ; and disannul, as an unauthorized and unnecessary substitute for annul. He prefers skeptic to sceptic ; gimlet to gimblet ; Mohammedism to Mohammedanism ; chamomile to camomile ; handcraft to handicraft ; handwork to handywork ; incase to encase ; enlist to inlist ; embody to imbody. ... On the principles laid down in the Preface to this Abridgment, most of these words were inserted under both their forms ; and still are suffered to stand because it was found difficult to make the change. It will be understood, however, from this statement, which form the author approves.
The dictionary itself retains the cross reference "SKEPTIC, See SCEPTIC" under the alphabetic entry for skeptic. But rather bizarrely when you turn to the place where you'd expect sceptic to appear—namely, between the entries for scentless and sceptre—you instead find full entries for skeptic, skeptical, skeptically, skepticalness, skepticism, and skepticize.
Finally, the "New Revised Edition" of 1847—the edition of the dictionary issued following the death of Webster and the acquisition of the rights to his dictionary by the G. & C. Merriam Company, moved the skeptic entries to their correct alphabetical location and signaled the demise of sceptic with the entry there of the simple cross reference
SCEPTIC. See SKEPTIC.
Conclusion
Whatever train of thought led Webster to abandon sceptic in favor of skeptic, it was by no means an original idea, as skeptic had begun appearing in English dictionaries for more than a century before Webster embraced it. Nevertheless, it seems extremely likely that the dominance of Merriam-Webster's dictionaries in the American market from 1850 forward contributed greatly to the ascendency of that spelling in the United States.
