Is it ensure, insure, or assure?
1650s, back-formation from obesity and in part from Latin obesus "fat, stout, plump," past participle of obedere "that has eaten itself fat" (see obesity). According to OED, "Rare before 19th c." Related: Obeseness. Latin obesus was translated in Old English as oferfæt "overfat."
obese o·bese (ō-bēs')
Extremely fat; very overweight.