The Collaborative International Dictionary of English v.0.48:

Leash \Leash\ (l[=e]sh), n. [OE. lese, lees, leece, OF. lesse,
   F. laisse, LL. laxa, fr. L. laxus loose. See Lax.]
   1. A thong of leather, or a long cord, by which a person may
      hold or restrain an animal, such as a falconer holding his
      hawk, or a courser his dog. For dogs and cats, the leash
      is commonly attached to a collar around the neck of the
      animal.
      [1913 Webster]

            Even like a fawning greyhound in the leash. --Shak.
      [1913 Webster]

   2. (Sporting) A brace and a half; a tierce; three; three
      creatures of any kind, especially greyhounds, foxes,
      bucks, and hares; hence, the number three in general.
      [1913 Webster]

            [I] kept my chamber a leash of days.  --B. Jonson.
      [1913 Webster]

            Then were I wealthier than a leash of kings.
                                                  --Tennyson.
      [1913 Webster]

   3. (Weaving) A string with a loop at the end for lifting warp
      threads, in a loom.
      [1913 Webster]

   To keep (a person) on a short leash to maintain close
      control over the activities of (a person).
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Leash \Leash\, v. t. [imp. & p. p. Leashed; p. pr. & vb. n.
   Leashing.]
   To tie together, or hold, with a leash.
   [1913 Webster]

WordNet (r) 3.0 (2006):

leash
    n 1: restraint consisting of a rope (or light chain) used to
         restrain an animal [syn: leash, tether, lead]
    2: the cardinal number that is the sum of one and one and one
       [syn: three, 3, III, trio, threesome, tierce,
       leash, troika, triad, trine, trinity, ternary,
       ternion, triplet, tercet, terzetto, trey, deuce-
       ace]
    3: a figurative restraint; "asked for a collar on program
       trading in the stock market"; "kept a tight leash on his
       emotions"; "he's always gotten a long leash" [syn: collar,
       leash]
    v 1: fasten with a rope; "rope the bag securely" [syn: rope,
         leash]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "leash":
   Oregon boat, anchor, band, bandage, belt, bend, bilbo, bind,
   bind up, bond, bonds, brace, bridle, bundle, camisole, chain,
   chains, cinch, clover, collar, cuffs, deuce-ace, do up, enchain,
   entrammel, fasten, fetter, gag, gird, girdle, girt, girth, gyve,
   gyves, halter, hamper, handcuff, handcuffs, hobble, hobbles,
   hog-tie, hopple, hopples, irons, lace, lash, leading strings,
   make fast, manacle, moor, muzzle, peg down, picket, pillory,
   pin down, pinion, put in irons, reins, restrain, restraint,
   restraints, rope, secure, set of three, shackle, shamrock, splice,
   stocks, straightjacket, strait-waistcoat, straitjacket,
   stranglehold, strap, swaddle, swathe, tercet, ternary, ternion,
   terzetto, tether, three, threesome, tie, tie down, tie up, tierce,
   trammel, trammels, trefoil, trey, triad, trialogue, triangle,
   tricorn, trident, triennium, trihedron, trilogy, trimester, trine,
   trinity, trinomial, trio, triphthong, triple crown, triple threat,
   triplet, triplopy, tripod, triptych, trireme, triseme, triskelion,
   trisul, triumvirate, triunity, trivet, troika, truss, wire, wrap,
   wrap up, yoke