Is it ensure, insure, or assure?
late 14c., "to lay (a crime, etc.) to the account of," from Old French imposer "put, place; impute, charge, accuse" (c.1300), from assimilated form of in- "into, in" (see in- (2)) + poser "put, place" (see pose (v.1)). Sense of "to lay on as a burden" first recorded 1580s. Related: Imposed; imposing.