Is it ensure, insure, or assure?
Old English wedlac "pledge-giving, marriage vow," from wed + -lac, noun suffix meaning "actions or proceedings, practice," attested in about a dozen Old English compounds (cf. feohtlac "warfare"), but this is the only surviving example. Suffix altered by folk etymology through association with lock (n.1). Meaning "condition of being married" is recorded from early 13c.