mid-15c., "small box for jewels, etc.," possibly a diminutive of English cask with -et, or from a corruption of Middle French casset "a casket, a chest" (see cassette). Also a publisher's name for a collection of selected literary or musical pieces (1828). Meaning "coffin" (especially an expensive one) is American English, probably euphemistic, attested by 1832.
Thank Heaven, the old man did not call them "CASKETS!"—a vile modern phrase, which compels a person of sense and good taste to shrink more disgustfully than ever before from the idea of being buried at all. [Hawthorne, "Our Old Home," 1862]