Old English dælere "divider, distributor; agent, negotiator," agent noun from deal (v.). Meaning "player who passes out the cards in a game" is from c. 1600; meaning "one whose business is to buy and sell merchandise" is from 1610s. Meaning "purveyor of illegal drugs" is recorded by 1920.
"dealer in ornamental feathers, one who prepares plumes for ornamental purposes," 1590s, from French plumassier, from plumasse "plume of feather," from plume (see plume (n.)). Earlier was plumer "dealer in feathers" (late 13c.).
also merchandiser, "a dealer in merchandise," 1590s, agent noun from merchandize (v.).
"bookseller," 1775, from Latin bibliopola, from Greek bibliopōlēs "bookseller," from biblion "book" (see biblio-) + pōlēs "merchant, seller," from pōlein "to sell" (from PIE root *pel- (4) "to sell"). Especially a dealer in rare or curious books. French has bouquinist "a dealer in second-hand books of little value."
"to cheat, defraud," 1560s, of uncertain origin; perhaps from French cousiner "cheat on pretext of being a cousin;" or from Middle English cosyn "fraud, trickery" (mid-15c.), which is perhaps related to Old French coçon "dealer, merchant, trader," from Latin cocionem "horse dealer." Related: Cozened; cozening; cozenage.