Is it ensure, insure, or assure?
mid-13c., probably originally "in season, timely, opportune, excellent," from tide in the sense of "season, time" (see tide). Cf. Old High German zitig, German zeitig, Dutch tijdig, Danish tidig "timely." Meaning "neat and in order" first recorded 1706.
"to make neat, set in order," 1821, from tidy (adj.). Related: Tidied; tidying.