Why is the ninth month called September?
mid-14c. "sell in small quantities or parcels," from Old French retaillier "cut back, cut off, pare, clip, reduce, circumcise," from re- "back" (see re-) + taillier "to cut, trim" (see tailor (n.)). Sometimes also "to deal out (information, etc.) in small quantities; hand down by report; recount, tell over again" (1590s). Related: Retailed; retailing.
early 15c., "sale of commodities in small quantities or parcels or at second hand" (opposed to wholesale), from Old French retail "piece cut off, shred, scrap, paring" (Modern French retaille), from retaillier (see retail (v.)). The notion of the English word is "a selling by the piece." This sense is not in French, however, and comes perhaps from cognate Italian ritaglio, which does have that sense. As an adjective, "of or pertaining to sale at retail," c.1600.