Is it ensure, insure, or assure?
1520s, "unlucky," from un- (1) "not" + fortunate. Infortunate in same sense is from late 14c. (along with a verb infortune "to render unhappy," and a noun meaning "bad luck). In late 18c.-early 19c., unfortunate woman was a polite way to say "prostitute." The noun meaning "one who is not fortunate" is recorded from 1630s.