late 14c., "path, track, course of action," introduced by the Hanse merchants, from Middle Dutch or Middle Low German trade "track, course" (probably originally of a ship), cognate with Old English tredan (see tread (v.)).
Sense of "one's habitual business" (1540s) developed from the notion of "way, course, manner of life" (mid-15c.); sense of "buying and selling, exchange of commodities" is from 1550s. Meaning "act of trading" is from 1829. Trade-name is from 1821; trade-route is from 1873; trade-war is from 1899. Trade union is attested from 1831. Trade wind (1640s) has nothing to do with commerce, but preserves the obsolete sense of "in a habitual or regular course."
1540s, "to tread a path," from trade (n.). Meaning "to occupy oneself (in something)" is recorded from c. 1600. Meaning "to barter" is by 1793. The U.S. sports team sense of "to exchange one player for another" is attested from 1899. Related: Traded; trading. To trade down is attested from 1942; trade up from 1959. Trade places "exchange situations" is from 1917. Trading post is recorded from 1796. Trading stamp, given by merchants and exchangeable for goods, is from 1897.