The Collaborative International Dictionary of English v.0.48:

Girth \Girth\, v. t. [From Girth, n., cf. Girt, v. t.]
   To bind as with a girth. [R.] --Johnson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Girth \Girth\ (g[~e]rth), n. [Icel. gj["o]r[eth] girdle, or
   ger[eth] girth; akin to Goth. ga['i]rda girdle. See Gird to
   girt, and cf. Girdle, n.]
   1. A band or strap which encircles the body; especially, one
      by which a saddle is fastened upon the back of a horse.
      [1913 Webster]

   2. The measure around any object, such as a body at the waist
      or belly, or a box; the circumference of anything; as, in
      order to be acceptable for mailing, the total of height
      and girth of a package must not exceed 63 inches.
      [1913 Webster +PJC]

            He's a lusty, jolly fellow, that lives well, at
            least three yards in the girth.       --Addison.
      [1913 Webster]

   3. A small horizontal brace or girder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

girth
    n 1: the distance around a person's body
    2: stable gear consisting of a band around a horse's belly that
       holds the saddle in place [syn: cinch, girth]
    v 1: tie a cinch around; "cinch horses" [syn: cinch, girth]

Moby Thesaurus II by Grady Ward, 1.0:

249 Moby Thesaurus words for "girth":
   ambit, amplitude, anchor, anklet, area, armlet, back band,
   backstrap, band, bandage, bar, barrette, bearing rein, bellyband,
   belt, bend, bigness, bind, bind up, binding, binding stone,
   binding twine, bit, blinders, blinds, bobby pin, body, bollard,
   bolt, bonder, bondstone, border, box hook, brace, bracelet, braces,
   brad, braid, breadth, breeching, bridle, buckle, bulk, bundle,
   button, cable, caliber, caparison, carpet tack, catch, cavesson,
   cestus, chain, checkrein, cheekpiece, chinband, cinch, cincture,
   cingulum, circuit, circumference, clamp, clasp, cleat, click,
   clinch, clip, clothespin, collar, collarband, corking pin, cotter,
   coverage, crownband, crupper, cummerbund, curb, depth, diameter,
   dimension, dimensions, do up, dowel, earring, ecliptic, equator,
   expanse, expansion, extension, extent, fascia, fibula, fillet,
   finger ring, fishhook, funiculus, gag swivel, garter, gauge, gird,
   girdle, girt, grab, grapnel, grappler, grappling iron,
   great circle, greatness, guy, hackamore, hairpin, halter, hames,
   hametugs, hank, harness, hasp, hawser, haywire, headgear,
   headstall, height, hip straps, hitch, hitching post, holdfast,
   hook, hook and eye, hoop, horn, inkle, interlocker, jaquima,
   jerk line, jockey, kevel, kingpin, lace, lacing, largeness, lariat,
   lash, latch, latchet, leader, leash, length, ligament, line, lines,
   lock, loop, magnitude, martingale, mass, measure, measurement,
   moorings, nail, neckband, necklace, noose, nose ring, noseband,
   nut, padlock, paper clip, pawl, peg, perimeter, periphery, pin,
   pintle, pole strap, pommel, proportion, proportions, quoit, radius,
   range, reach, reins, ribbons, ring, rivet, roller, rope, saddle,
   safety pin, scale, scope, screw, seal, sennit, setscrew, shaft tug,
   side check, size, skewer, slide fastener, snaffle, snap,
   snubbing post, spike, splice, spread, staple, stirrup, strap,
   string, strop, stub tenon, surcingle, suspenders, swaddle, swathe,
   tack, tackle, tag, tendon, terret, thole, tholepin, thong,
   thumbtack, tie, tie beam, tie up, toggle, trappings, treenail,
   truss, tug, twine, vise, volume, waistband, whang, width,
   winker braces, wire, wrap, wrap up, wristband, wristlet, yoke,
   zipper, zodiac, zone



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

GIRTH., A girth or yard is a measure of length. The word is of Saxon origin,
taken from the circumference of the human body. Girth is contracted from
girdeth, and signifies as much as girdle. See Ell.