Is it ensure, insure, or assure?
abstract suffix of state, from Old English dom "statute, judgment" (see doom (n.)). Already active as a suffix in Old English (e.g. freodom, wisdom); from stem *do- "do" + *-moz abstract suffix. Cf. cognate German -tum, Old High German tuom.
used to form nouns The range, establishment, scope, or realm of what is indicated: fandom/ moviedom/ klutzdom