Is it ensure, insure, or assure?
Old English merran (Anglian), mierran (West Saxon) "to waste, spoil," from Proto-Germanic *marzjan (cf. Old Frisian meria, Old High German marren "to hinder, obstruct," Gothic marzjan "to hinder, offend"), from PIE root *mers- "to trouble, confuse" (cf. Sanskrit mrsyate "forgets, neglects," Lithuanian mirszati "to forget"). Related: Marred; marring.