also coigne, an archaic spelling of quoin (q.v.) the survival of which is due to Shakespeare's coign of vantage ("Macbeth" I.vi.), popularized by Sir Walter Scott; in this phrase it is properly "a projecting corner" (for observation).
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 Latin ad- words. Meaning "any condition favorable to success, a favoring circumstance" (the opposite of a disadvantage) is from late 15c. Tennis score sense is from 1640s (in the writings of John Milton). Phrase to 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.