Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. The measure round the body, as at the waist or belly; the
      circumference of anything.

            He's a lu sty, jolly fellow, that lives well, at
            least three yards in the girth.       --Addison.

   3. A small horizontal brace or girder.

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

Source : WordNet®

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}]
     v 1: encircle or bind; "Trees girded the green fields" [syn: {girt},
           {begird}, {gird}]
     2: tie a cinch around; "cinch horses" [syn: {cinch}]