The Collaborative International Dictionary of English v.0.48:

Tack \Tack\, n. [OE. tak, takke, a fastening; akin to D. tak a
   branch, twig, G. zacke a twig, prong, spike, Dan. takke a
   tack, spike; cf. also Sw. tagg prickle, point, Icel. t[=a]g a
   willow twig, Ir. taca a peg, nail, fastening, Gael. tacaid,
   Armor. & Corn. tach; perhaps akin to E. take. Cf. Attach,
   Attack, Detach, Tag an end, Zigzag.]
   1. A small, short, sharp-pointed nail, usually having a
      broad, flat head.
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   2. That which is attached; a supplement; an appendix. See
      Tack, v. t., 3. --Macaulay.
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            Some tacks had been made to money bills in King
            Charles's time.                       --Bp. Burnet.
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   3. (Naut.)
      (a) A rope used to hold in place the foremost lower
          corners of the courses when the vessel is closehauled
          (see Illust. of Ship); also, a rope employed to pull
          the lower corner of a studding sail to the boom.
      (b) The part of a sail to which the tack is usually
          fastened; the foremost lower corner of fore-and-aft
          sails, as of schooners (see Illust. of Sail).
      (c) The direction of a vessel in regard to the trim of her
          sails; as, the starboard tack, or port tack; -- the
          former when she is closehauled with the wind on her
          starboard side; hence, the run of a vessel on one
          tack; also, a change of direction; as, to take a
          different tack; -- often used metaphorically.
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   4. (Scots Law) A contract by which the use of a thing is set,
      or let, for hire; a lease. --Burrill.
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   5. Confidence; reliance. [Prov. Eng.] --Halliwell.
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   Tack of a flag (Naut.), a line spliced into the eye at the
      foot of the hoist for securing the flag to the halyards.


   Tack pins (Naut.), belaying pins; -- also called jack
      pins.

   To haul the tacks aboard (Naut.), to set the courses.

   To hold tack, to last or hold out. --Milton.
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