Is it ensure, insure, or assure?
past participle of do; from Old English past participle gedon (a vestige of the prefix is in ado). U.S. Southern use of done in phrases like "he done gone to the store" is attested from 1827, according to OED: "a perfective auxiliary or with adverbial force in the sense 'already; completely.' " Meaning "finished" is early 15c. Slang done for "doomed" is from 1842.