Is it ensure, insure, or assure?
early 13c., from un- (1) "not" + past participle of seek. Cf. Middle Dutch ongesocht, Dutch ongezocht, Middle High German ungesuochet, German ungesucht.
past tense and past participle of seek, from Old English sohte. Sought-after is from 1881 (sought-for in same sense is from c.1600).