Is it ensure, insure, or assure?
late 14c., from Old French desmerite "blame, demerit" (Modern French démérite), from des- "not, opposite" (see dis-) + merite "merit" (see merit (n.)). Latin demereri meant "to merit, deserve," from de- in its completive sense. But Medieval Latin demeritum meant "fault." Both senses existed in the Middle French form of the word. Meaning "penalty point in school" is attested from 1862.