[This is a fast-access FAQ excerpt.]
This word, meaning "extremely satisfactory", was first recorded
in 1919, and was originally heard chiefly among U.S. black jazz
musicians. The tap dancer Bill "Bojangles" Robinson (1878-1949)
popularized the word, and claimed to have coined it when he was a
shoeshine boy in Richmond; but a number of Southerners testified
that they had heard the word used by parents or grandparents in the
late 19th century. Suggested origins include: a supposed Italian
word copacetti; a Creole French word coupersetique meaning "that
can be coped with"; and the Hebrew phrase kol besedeq "all with
justice". RHUD2 says that all these theories "lack supporting
evidence".