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"clasp, hook, fastener," also "a nail of some kind," late 13c., from Old North French taque "nail, pin, peg," probably from a Germanic source (cf. Middle Dutch tacke "twig, spike," Low German takk "tine, pointed thing," German Zacken "sharp point, tooth, prong"); perhaps related to tail. 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 14c.
"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," 1833, perhaps a shortening and special use of tackle (n.) in the sense of "gear."
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.
"sail into the wind," 1550s, from tack (n.1) in the sailing sense. Figurative sense of "course or line of conduct or action" is from 1670s. Related: Tacked; tacking.