Is it ensure, insure, or assure?
c.1200, "track or trail of a person," from Old Norse sloð "trail," of uncertain origin. Meaning "detective" is 1872, shortening of sleuth-hound "keen investigator" (1849), a figurative use of a word that dates back to late 14c. meaning a kind of bloodhound. The verb (intransitive) meaning "to act as a detective, investigate" is recorded from 1905. Related: Sleuthed; sleuthing.