disadvantage 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).

early 14c., avantage, "position of being in advance of another," from Old French avantage "advantage, profit; superiority" (12c.), from avant "before," probably via an unrecorded Late or Medieval Latin *abantaticum, from Latin abante "from before," composed of ab "from" (see ab-) + ante "before, in front of, against" (from PIE root *ant- "front, forehead"). Compare advance (v.).

Advantage is the possession of a good vantage-ground for the attainment of ulterior objects of desire .... [Century Dictionary]

The unetymological -d- is a 16c. intrusion on the analogy of the many Latin ad- words in English. The meaning "any condition favorable to success, a favoring circumstance" (the opposite of a disadvantage) is from late 15c. The tennis score sense is from 1640s (in the writings of John Milton). Phrase take advantage of is from late 14c. as "avail oneself of," also "impose upon." To have the advantage of (someone) "have superiority over" is from 1560s.