Is it ensure, insure, or assure?
1620s, auf, oph (modern form from 1630s), "a changeling; a foolish child left by the fairies" [Johnson], from a Scandinavian source, cf. Norwegian alfr "silly person," in Old Norse, "elf" (see elf). Hence, "a misbegotten, deformed idiot." Until recently, some dictionaries still gave the plural as oaves.