demerityoudaoicibaDictYouDict[demerit 词源字典]
demerit: [14] A demerit may be virtually the opposite of a merit, but the word was not formed, as might be supposed, by adding the prefix de-, denoting oppositeness or reversal, to merit. Its distant ancestor was Latin demeritum, from the verb demereri ‘deserve’, where the de- prefix meant not ‘opposite of’ but ‘completely’ (as it does too in, for example, denude and despoil).

Add this de- to mereri ‘deserve’ and you get ‘deserve thoroughly’. However, at some point in the Middle Ages the prefix began to be reinterpreted as ‘opposite’, and medieval Latin demeritum came to mean ‘fault’ – the sense that reached English via French démérite.
[demerit etymology, demerit origin, 英语词源]
demerit (n.)youdaoicibaDictYouDict
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.