1. Chief; eminent; greatest; principal.
"The most arch act of piteous massacre." Shakespeare
--Merriam-Webster's Unabridged Dictionary of the English Language Second Ed., 1955 [aka Webster 2]
archly, adv. archness n.
[arch-, originally in arch rogue etc.]
Ety: Ultimately from the Latin arcus arc.
--The Concise Oxford Dictionary of Current English, 1995.
THE WORD IN USE (as adv.):
From "The Courtship of Miles Standish," iii,
Henry Wadsworth Longfellow, 1858.
Archly the maiden smiled, and, with eyes overrunning with laughter,
Said, in a tremulous voice, 'Why don't you speak for yourself, John?
--The Oxford Dictionary of Quotations, Second Edition, 1955.
*archwise: a nonce word° invented by B'n'J'n, meaning able to understand and express an arch attitude; hip; savvy; "in the groove"
°nonce word = a word (as ringday in "four girls I know have become engaged today: this must be ringday") coined and used apparently to suit one particular occasion sometimes independently by different writers or speakers but not adopted into use generally.--Webster 3.
*Yes, the Puritans were known to flirt.
AN EMENDATION (9 FEB, 2007):
I realized, just yesterday, that, with an arch attitude, comes an arch look to the face, and with an arch look to the face often comes an arched eyebrow--which, it strikes me now, is a most apt, emblematic image to use in completing our discussion of arch in its mischievous sense.
BTW, I tried arching each of my eyebrows without bringing the other into play, but I couldn't do it. How about you, dear reader? Can you arch one eyebrow without any squinting or bending on the other side?