Is it ensure, insure, or assure?
mid-14c., "iron cross-bar of a window," from Old French loquet "door-handle, bolt, latch," diminutive of loc "lock, latch," from Frankish or some other Germanic source (cf. Old Norse lok "fastening, lock;" see lock (n.1)). Meaning "ornamental case with hinged cover" (containing a lock of hair, miniature portrait, etc.) first recorded 1670s.