demerit n.

word-forming element of Latin origin meaning 1. "lack of, not" (as in dishonest); 2. "opposite of, do the opposite of" (as in disallow); 3. "apart, away" (as in discard), from Old French des- or directly from Latin dis- "apart, asunder, in a different direction, between," figuratively "not, un-," also "exceedingly, utterly." Assimilated as dif- before -f- and to di- before most voiced consonants.

The Latin prefix is from PIE *dis- "apart, asunder" (source also of Old English te-, Old Saxon ti-, Old High German ze-, German zer-). The PIE root is a secondary form of *dwis- and thus is related to Latin bis "twice" (originally *dvis) and to duo, on notion of "two ways, in twain" (hence "apart, asunder").

In classical Latin, dis- paralleled de- and had much the same meaning, but in Late Latin dis- came to be the favored form and this passed into Old French as des-, the form used for compound words formed in Old French, where it increasingly had a privative sense ("not"). In English, many of these words eventually were altered back to dis-, while in French many have been altered back to de-. The usual confusion prevails.

As a living prefix in English, it reverses or negatives what it is affixed to. Sometimes, as in Italian, it is reduced to s- (as in spend, splay, sport, sdain for disdain, and the surnames Spencer and Spence).

c. 1200, "spiritual credit" (for good works, etc.); c. 1300, "spiritual reward," from Old French merite "wages, pay, reward; thanks; merit, moral worth, that which assures divine pity" (12c.) and directly from Latin meritum "a merit, service, kindness, benefit, favor; worth, value, importance," neuter of meritus, past participle of merere, mereri "to earn, deserve, acquire, gain," from PIE root *(s)mer- (2) "to get a share of something."

Sense of "worthiness, excellence," is from early 14c.; from late 14c. as "state or fact of deserving, condition or conduct that deserves either reward or punishment;" also "a reward, benefit." Etymologically it is merely "that which one deserves," and the Latin word was used of rewards or punishments, but in English it has typically meant "state or fact of deserving well."

Merits, in law, is "the right and wrong of the case, essential facts and principles" (as distinguished from questions of procedure, etc.). In civil service promotion, the merit system is attested by 1880 (opposed to the spoils system); the phrase was used earlier in other contexts. Merit-monger (1550s, Latimer) was a common 16c.-17c. term of theological contempt for one who believes that human merit entitles man to divine rewards.

Proto-Indo-European root meaning "to get a share of something." 

It forms all or part of: demerit; emeritus; isomer; isomeric; meretricious; merism; meristem; merit; meritorious; mero-; monomer; Moira; polymer; turmeric.

It is the hypothetical source of/evidence for its existence is provided by: Greek meros "part, lot," moira "share, fate," moros "fate, destiny, doom;" Hittite mark "to divide" a sacrifice; Latin merere, meriri "to earn, deserve, acquire, gain."