First in print in 1919, Bachelles, "Man For The Ages." "As to looks, I'd call him, as ye might say, real copacetic." Mrs Lukins expressed this opinion solemnly. Its last word stood for nothing more than an indefinite depth of meaning." OED, 1987 supplement.
In 1926, Van Vechten used it in "Nigger Heaven." An aprobatory epithet somewhat stronger than all right. Also OED, 1987 supplement.