mid-14c. (mid-13c. in surnames), "commercial agent, factor," also "an agent in sordid business," from Anglo-French brocour "small trader," from abrokur "retailer of wine, tapster;" perhaps from Portuguese alborcar "barter," but more likely from Old French brocheor, from brochier "to broach, tap, pierce (a keg)," from broche (Old North French broke, broque) "pointed tool" (see broach (n.)), with an original sense of "wine dealer," hence "retailer, middleman, agent." In Middle English, used contemptuously of peddlers and pimps, "one who buys and sells public office" (late 14c. in Anglo-French), "intermediary in love or marriage" (late 14c.).
1660s, "to buy and sell as a broker" (intransitive), from job (n.). Meaning "deal in public stocks on one's own account" is from 1721. Meaning "to cheat, betray" is from 1903; earlier "pervert public service to private advantage" (1732). Related: Jobbed; jobbing.
1853, "charge made or percentage received by a broker or seller for deferring settlement of a stock sale," a stockbroker's invention, perhaps somehow derived from continue, or from Spanish contengo "I contain, refrain, restrain, check." Continuation was used in this sense from 1813. As a verb, from 1900.