The Collaborative International Dictionary of English v.0.48:

Cinch \Cinch\, v. t.
   In the game of cinch, to protect (a trick) by playing a
   higher trump than the five.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Cinch \Cinch\, n. [Sp. cincha, fr. L. cingere to gird.]
   [1913 Webster]
   1. A strong saddle girth, as of canvas. [West. U. S.]
      [1913 Webster]

   2. A tight grip. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cinch \Cinch\, v. t. [imp. & p. p. Cinched; p. pr. & vb. n.
   Cinch"ing.]
   1. To put a cinch upon; to girth tightly. [Western U. S.]
      [Webster 1913 Suppl.]

   2. To get a sure hold upon; to get into a tight place, as for
      forcing submission. [Slang, U. S.]
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Cinch \Cinch\, v. i.
   To perform the action of cinching; to tighten the cinch; --
   often with up. [Western U. S.]
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Cinch \Cinch\, n. [Cf. cinch a girth, a tight grip, as v., to
   get a sure hold upon; perh. so named from the tactics used in
   the game; also cf. Sp. cinco five (the five spots of the
   color of the trump being important cards).]
   A variety of auction pitch in which a draw to improve the
   hand is added, and the five of trumps (called right pedro)
   and the five of the same color (called left pedro, and
   ranking between the five and the four of trumps) each count
   five on the score. Fifty-one points make a game. Called also
   double pedro and high five.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cinch
    n 1: any undertaking that is easy to do; "marketing this product
         will be no picnic" [syn: cinch, breeze, picnic,
         snap, duck soup, child's play, pushover,
         walkover, piece of cake]
    2: stable gear consisting of a band around a horse's belly that
       holds the saddle in place [syn: cinch, girth]
    3: a form of all fours in which the players bid for the
       privilege of naming trumps
    v 1: tie a cinch around; "cinch horses" [syn: cinch, girth]
    2: make sure of
    3: get a grip on; get mastery of

Moby Thesaurus II by Grady Ward, 1.0:

198 Moby Thesaurus words for "cinch":
   affix, afford proof of, anchor, annex, ascertain, assure, attach,
   babe, back band, backstrap, band, bandage, bearing rein, belay,
   bellyband, belt, bend, bind, bind up, bit, blinders, blinds, boob,
   brace, breeching, breeze, bridle, bring home to, bundle, caparison,
   cavesson, cement, certify, chain, checkrein, cheekpiece, chinband,
   chump, clamp, clear up, clinch, collar, cramp, credulous person,
   crownband, crupper, cull, curb, dead-sure thing, decide,
   demonstrate, determine, dismiss all doubt, do up, dupe, easy mark,
   easy pickings, easy target, easy thing, engraft, ensure, establish,
   fall guy, fasten, find out, fish, fix, follow, follow from, fool,
   gag swivel, get at, gird, girdle, girt, girth, gobe-mouches, graft,
   grapple, greener, greenhorn, greeny, gudgeon, gull, hackamore,
   halter, hames, hametugs, harness, have a case, headgear, headstall,
   hip straps, hold good, hold water, horn, innocent, insure, jaquima,
   jerk line, jockey, knit, lace, lash, lead-pipe cinch,
   leadpipe cinch, leash, lines, make certain, make fast, make good,
   make no doubt, make no mistake, make out, make sure, make sure of,
   martingale, monkey, moor, nail down, natural, noseband,
   open-and-shut case, patsy, picnic, pie, piece of cake, pigeon,
   pipe, plaything, pole strap, pommel, prize sap, prove, prove to be,
   prove true, pushover, put to, reassure, reins, remove all doubt,
   ribbons, rope, saddle, sap, saphead, schlemiel, screw up, secure,
   see that, see to it, set, set at rest, set to, settle,
   settle the matter, setup, shaft tug, shoo-in, show, side check,
   sinecure, sitting duck, snaffle, snap, sort out, splice, stirrup,
   stooge, strap, sucker, surcingle, sure bet, sure card,
   sure success, sure thing, swaddle, swathe, tack, tackle, tie,
   tie up, tighten, toy, trappings, trice up, trim, truss,
   trusting soul, tug, velvet, victim, winker braces, winner, wire,
   wrap, wrap up, yoke