Is it ensure, insure, or assure?
c.1300, from stem of Old French joindre "join, connect, unite; have sexual intercourse with" (12c.), from Latin iungere "to join together, unite, yoke," from PIE *yeug- "to join, unite" (see jugular). Related: Joined; joining. In Middle English, join sometimes is short for enjoin. Join up "enlist in the army" is from 1916. Phrase if you can't beat them, join them is from 1953.