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."
also trade-mark, 1838 (the thing itself attested continuously from 14c., apparently originally the watermarks on paper), from trade (n.) + mark (n.1) in a specialized sense of "stamp, seal, brand, etc. placed upon an article top indicate ownership or origin" (mid-13c.). Figurative use by 1869. As a verb, from 1904. Related: Trademarked; trademarking. This sense of mark also yielded the meaning "particular brand or make of an article" (1660s), hence its use in 20c. names of cars, etc., Mark I, Mark II, etc.