Is it ensure, insure, or assure?
1780 (transitive); 1792 (intransitive), a nautical word of obscure origin, perhaps (as Skeat suggests) from Spanish capuzar "to sink by the head," from cabo "head," from Latin caput (see capitulum). For sense, cf. French chavirer "to capsize, upset," faire capot "capsize;" Provençal cap virar "to turn the head." Related: Capsized; capsizing.