Green’s Dictionary of Slang

copacetic adj.

also copa, copasetic, copasetty, copesette(e), copissettic, copus, kopacetic, kopasetic, kopasetee, kopasette
[? Chinook jargon copasenee, everything is satisfactory, esp. as orig. used on the waterways of Washington state. Other etys. include: (i) the painfully contrived phr. the cop is on the settee, i.e. the cop is not paying attention, which elided into copacetic and was supposedly used as such by US hoodlums; (ii) a word presumed to be Ital. but otherwise unknown; (iii) Fr. coupersetique, f. couper, to strike; thus striking or worth a strike; (iv) the Yid. phr. hakol b’seder, all is in order or, earlier, kol b’tzedek, all with justice. Note that HDAS dismisses all these and states ‘ety. unknown’]

[1910s+] (US) excellent, first-rate; OK, satisfactory.