Is it ensure, insure, or assure?
early 14c., hathir, from Old English *hæddre, Scottish or northern England dialect name for Calluna vulgaris, probably altered by heath, but real connection to that word is unlikely [Liberman, OED]. Perhaps originally Celtic. As a fem. proper name little used in U.S. before 1935, but a top-15 name for girls born there 1971-1989.