Is it ensure, insure, or assure?
late 14c., "to withhold oneself," from Old French abstenir (14c.), earlier astenir (13c.) "hold (oneself) back, refrain, abstain (from), practice abstinence," from Latin abstinere "withhold, keep back, keep off," from ab(s)- "from, away from" (see ab-) + tenere "to hold" (see tenet). Specifically of liquor, attested from late 14c. Of voting, 1796. Related: Abstained; abstaining.