Is it ensure, insure, or assure?
c.1300, confyrmacyoun, the Church rite, from Old French confirmacion (13c.) "strengthening, confirmation; proof; ratification," from Latin confirmationem (nominative confirmatio) "a securing, establishing; an assurance, encouragement," noun of action from confirmare (see confirm). As a legal action, "verification, proof," from late 14c.; as "action of making sure," from late 15c.