Is it ensure, insure, or assure?
c.1200, dauen, "to dawn, grow light," shortened or back-formed from dauinge, dauing "period between darkness and sunrise," (c.1200), from Old English dagung, from dagian "to become day," from root of dæg "day" (see day). Probably influenced by a Scandinavian word (cf. Danish dagning, Old Norse dagan "a dawning;" cf. also German tagen "to dawn"). Related: Dawned; dawning.
1590s, from dawn (v.).