c.1200, "track or trail of a person," from O.N. sloð "trail," of uncertain origin. Meaning "detective" is 1872, shortening of sleuthhound "keen investigator" (1849), a figurative use of a word for a kind of bloodhound that dates back to late 14c. The verb (intrans.) meaning "to act as a detective, investigate" is recorded from 1912.