"clasp, hook, fastener," also "a nail" of some kind, c.1400, from Old North French taque "nail, pin, peg" (Old French tache, 12c., "nail, spike, tack; pin brooch"), probably from a Germanic source (compare Middle Dutch tacke "twig, spike," Frisian tak "a tine, prong, twig, branch," Low German takk "tine, pointed thing," German Zacken "sharp point, tooth, prong"), from Proto-Germanic *tag-. Meaning "small, sharp nail with a flat head" is attested from mid-15c. The meaning "rope to hold the corner of a sail in place" is first recorded late 15c.

"horse's harness, etc.," 1924, shortening of tackle, n. in sense of "equipment." Tack in a non-equestrian sense as a shortening of tackle is recorded in dialect from 1777.

"food" in general, but in dialect especially "bad food," and especially among sailors "food of a bread kind," 1833, perhaps a shortening and special use of tackle, n. in the sense of "gear." But compare tack "taste" (c.1600), perhaps a variant of tact.

late 14c., "to attach" with a nail, etc., from tack, n.1. Meaning "to attach as a supplement" (with suggestion of hasty or arbitrary proceeding) is from 1680s. Related: Tacked; tacking.

"turn a ship's course toward the wind at an angle," 1550s, from tack, n.1 in the ship-rigging sense (the ropes were used to move the vessel temporarily to one side or another of its general line of course, to take advantage of a side-wind); hence tack (n.) "course of conduct or mode of action suited to some purpose" (1670s), from figurative use of the verb (1630s). Related: Tacked; tacking.