Is it ensure, insure, or assure?
1753, originally nautical, also close-fights, "bulkheads fore and aft for men to stand behind in close engagements to fire on the enemy," it reflects the confusion of close (v.) and close (adj.); "now understood of proximity, but orig. 'closed' space on ship-board where last stand could be made against boarders" [Weekley]. Cf. also closed-minded, a variant of close-minded attested from 1880s, with a sense of "shut" rather than "tight."