Is it ensure, insure, or assure?
mid-15c., "small box for jewels, etc.," possibly a diminutive of English cask, or from a corruption of Middle French casset (see cassette). Meaning "coffin" is American English, probably euphemistic, attested by 1832.
Caskets! a vile modern phrase, which compels a person ... to shrink ... from the idea of being buried at all. [Hawthorne, "Our Old Home," 1863]