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 trading ship), cognate with Old English tredan (see tread). Sense of "one's habitual business" (1540s) developed from the notion of "way, course, manner of life" (mid-15c.); sense of "buying and selling" is first recorded 1550s. Trade wind (1640s) has nothing to do with commerce, but preserves the obsolete sense of "in a habitual or regular course." Trade union is attested from 1831.