Is it ensure, insure, or assure?
late 14c., "to reject another's authority as prejudiced," from Old French recuser (13c.), from Latin recusare "make an objection against; decline, refuse, reject; be reluctant to," from re- (see re-) + causa (see cause (n.)). Specifically, in law, "reject or challenge (a judge or juror) as disqualified to act." The word now is used mostly reflectively. Related: Recused; recusing.