The Collaborative International Dictionary of English v.0.48:

Tack \Tack\, n. [From an old or dialectal form of F. tache. See
   Techy.]
   1. A stain; a tache. [Obs.]
      [1913 Webster]

   2. [Cf. L. tactus.] A peculiar flavor or taint; as, a musty
      tack. [Obs. or Colloq.] --Drayton.
      [1913 Webster]

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.
      [1913 Webster]

   2. That which is attached; a supplement; an appendix. See
      Tack, v. t., 3. --Macaulay.
      [1913 Webster]

            Some tacks had been made to money bills in King
            Charles's time.                       --Bp. Burnet.
      [1913 Webster]
      [1913 Webster]

   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.
          [1913 Webster]

   4. (Scots Law) A contract by which the use of a thing is set,
      or let, for hire; a lease. --Burrill.
      [1913 Webster]

   5. Confidence; reliance. [Prov. Eng.] --Halliwell.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tack \Tack\, v. i. (Naut.)
   To change the direction of a vessel by shifting the position
   of the helm and sails; also (as said of a vessel), to have
   her direction changed through the shifting of the helm and
   sails. See Tack, v. t., 4.
   [1913 Webster]

         Monk, . . . when he wanted his ship to tack to
         larboard, moved the mirth of his crew by calling out,
         "Wheel to the left."                     --Macaulay.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tack \Tack\, v. t. [imp. & p. p. Tacked; p. pr. & vb. n.
   Tacking.] [Cf. OD. tacken to touch, take, seize, fix, akin
   to E. take. See Tack a small nail.]
   1. To fasten or attach. "In hopes of getting some commendam
      tacked to their sees." --Swift.
      [1913 Webster]

            And tacks the center to the sphere.   --Herbert.
      [1913 Webster]

   2. Especially, to attach or secure in a slight or hasty
      manner, as by stitching or nailing; as, to tack together
      the sheets of a book; to tack one piece of cloth to
      another; to tack on a board or shingle; to tack one piece
      of metal to another by drops of solder.
      [1913 Webster]

   3. In parliamentary usage, to add (a supplement) to a bill;
      to append; -- often with on or to; as, to tack on a
      non-germane appropriation to a bill. --Macaulay.
      [1913 Webster]

   4. (Naut.) To change the direction of (a vessel) when sailing
      closehauled, by putting the helm alee and shifting the
      tacks and sails so that she will proceed to windward
      nearly at right angles to her former course.
      [1913 Webster]

   Note: In tacking, a vessel is brought to point at first
         directly to windward, and then so that the wind will
         blow against the other side.
         [1913 Webster]

WordNet (r) 3.0 (2006):

tack
    n 1: the heading or position of a vessel relative to the trim of
         its sails
    2: a short nail with a sharp point and a large head
    3: gear for a horse [syn: stable gear, saddlery, tack]
    4: (nautical) a line (rope or chain) that regulates the angle at
       which a sail is set in relation to the wind [syn: sheet,
       tack, mainsheet, weather sheet, shroud]
    5: (nautical) the act of changing tack [syn: tack, tacking]
    6: sailing a zigzag course
    v 1: fasten with tacks; "tack the notice on the board"
    2: turn into the wind; "The sailors decided to tack the boat";
       "The boat tacked" [syn: tack, wear round]
    3: create by putting components or members together; "She pieced
       a quilt"; "He tacked together some verses"; "They set up a
       committee" [syn: assemble, piece, put together, set
       up, tack, tack together] [ant: break apart, break
       up, disassemble, dismantle, take apart]
    4: sew together loosely, with large stitches; "baste a hem"
       [syn: baste, tack]
    5: fix to; attach; "append a charm to the necklace" [syn:
       append, tag on, tack on, tack, hang on]
    6: reverse (a direction, attitude, or course of action) [syn:
       interchange, tack, switch, alternate, flip, flip-
       flop]

Moby Thesaurus II by Grady Ward, 1.0:

306 Moby Thesaurus words for "tack":
   MO, aberrancy, aberration, about ship, affix, algorithm, alter,
   alteration, ameliorate, annex, append, approach, articulate,
   attach, attack, azimuth, back and fill, back band, backstrap,
   baste, batten, batten down, be changed, be converted into,
   be renewed, bear away, bear off, bear to starboard, bearing,
   bearing rein, beat, beat about, bellyband, bend, bias, bind, bit,
   blinders, blinds, bolt, bottom out, box off, branch off,
   branching off, braze, break, breeching, bridle, bring about,
   bring round, buckle, butt, button, cant, cant round, caparison,
   cast, cast about, cavesson, cement, change, change course,
   change the bearing, change the heading, checker, checkrein,
   cheekpiece, chinband, chop, chop and change, cinch, circuitousness,
   clasp, cleat, clip, collar, combine, come about, come around,
   come round, compass bearing, compass direction, cordage, cording,
   corner, couple, course, crook, crownband, crupper, curb, curve,
   declination, deflection, degenerate, depart from, departure,
   deteriorate, detour, deviance, deviancy, deviate, deviation,
   deviousness, digress, digression, direction, discursion, divagate,
   divagation, divaricate, divarication, diverge, divergence,
   diversify, diversion, dogleg, double, double a point, dovetail,
   drift, drifting, equipage, equipment, errantry, excursion,
   excursus, exorbitation, fashion, fasten, fastening, fetch about,
   fittings, fix, flop, form, gag swivel, gear, girth, glue, go about,
   guise, gybe, hackamore, hairpin, halter, hames, hametugs, harness,
   hasp, haul around, headgear, heading, headstall, heave round, heel,
   hinge, hip straps, hitch, hook, improve, indirection, jam, jaquima,
   jerk line, jibe, jibe all standing, join, joint, kit, latch,
   lee side, line, line of action, lines, lock, magnetic bearing,
   manner, manner of working, martingale, means, meliorate, method,
   methodology, miss stays, miter, mitigate, mode, mode of operation,
   mode of procedure, modulate, modus operandi, mortise, mutate, nail,
   noseband, obliquity, order, outfit, paste, path, peg, pererration,
   pin, ply, pole strap, practice, procedure, proceeding, process,
   put about, put back, rabbet, rambling, reins, relative bearing,
   revive, ribbons, rig, rigging, rivet, ropework, roping,
   round a point, routine, saddle, saddlery, scarf, screw, secure,
   sew, shaft tug, sheer, shift, shifting, shifting course,
   shifting path, side check, skew, skewer, slant, slew, snaffle,
   snap, solder, staple, stick, stitch, straying, style, surcingle,
   sweep, swerve, swerving, swing round, swing the stern, swinging,
   system, tack on, tackle, tag on, take a turn, tangent, technique,
   the drill, the how, the way of, throw about, thumbtack, tie,
   toggle, tone, trappings, trend, true bearing, tug, turn,
   turn aside, turn back, turn into, turn the corner, turning, twist,
   undergo a change, unite, variation, vary, vector, veer, veer off,
   wandering, warp, way, wear, wear ship, weather side, wedge, weld,
   wind, winker braces, wise, worsen, yaw, yoke, zigzag, zipper



Bouvier's Law Dictionary, Revised 6th Ed (1856):

TACK, Scotch law. A contract of location by which the use of land, or any
other immovable subject, is, set to the lessee or tacksman for a certain
yearly rent, either in money, the fruits of tho ground, or services. Ersk.
Prin. Laws of Scot. B. 2, t. 6, n. 8; 1 Tho. Co. Litt. 209. This word is
nearly synonymous with lease.