Is it ensure, insure, or assure?
c.1600, from Italian baritono, from Greek barytonos "deep-toned, deep-sounding," from barys "heavy, deep," also, of sound, "strong, deep, bass" (see grave (adj.)) + tonos "tone" (see tenet). Technically, "ranging from lower A in bass clef to lower F in treble clef." Meaning "singer having a baritone voice" is from 1821. As a type of brass band instrument, it is attested from 1949.