Is it ensure, insure, or assure?
c.1300, "charge (with an offense, etc.), impugn, blame," from Old French acuser "to accuse, indict, reproach, blame" (13c.), earlier "announce, report, disclose" (12c.), or directly from Latin accusare "to call to account," from ad- "against" (see ad-) + causari "give as a cause or motive," from causa "reason" (see cause (n.)). Related: Accused; accusing; accusingly.