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]