I once heard the late John Ciardi (NPR's "A Word in Your Ear") try to explain that the 1920s idiom, "copacetic" (meaning completely satisfactory), entered into the African-American vocabulary in Harlem from the days when Jews and African Americans lived there together. He argued that copacetic has the same meaning as the Israeli idiom "kol b'seder" which literally means "all is in order." The problem with that, said my Harlem-raised father-in-law, is that the Jews in Harlem spoke Yiddish and kol b'seder was not used in Yiddish. The dictionary I've got is not helpful. Can someone come up with a better explanation?
The etymology is disputed and linguists do not seem to have a consensus.
Michael Quinion lists a number of implausible reasons and finally ends it with the first clear sighting of the word. This seems to be the closest we could get to a verifiable origin.
“Now there’s the kind of a man! Stout as a buffalo an’ as to looks I’d call him, as ye might say, real copasetic.” Mrs. Lukins expressed this opinion solemnly and with a slight cough. Its last word stood for nothing more than an indefinite depth of meaning.
A Man for the Ages, by Irving Bacheller, 1919.