Casket
late Middle English: perhaps an Anglo-Norman French form of Old French cassette, diminutive of casse (see case2).
wiktionary
Probably from Norman cassette. Possibly reformed by analogy with cask [1] [2], thus analyzable as cask + -et. Doublet of cassette.
etymonline
casket (n.)
mid-15c., "small box for jewels, etc.," possibly a diminutive of English cask with -et, or from a corruption of 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]
