The Collaborative International Dictionary of English v.0.48:

Bough \Bough\, n. [OE. bogh, AS. b[=o]g, b[=o]h, bough,
   shoulder; akin to Icel. b[=o]gr shoulder, bow of a ship, Sw.
   bog, Dan. bov, OHG. buog, G. bug, and to Gr.? ( for ? )
   forearm, Skr. b[=a]hu (for bh[=a]ghu) arm. [root]88, 251. Cf.
   Bow of a ship.]
   [1913 Webster]
   1. An arm or branch of a tree, esp. a large arm or main
      branch.
      [1913 Webster]

   2. A gallows. [Archaic] --Spenser.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bough
    n 1: any of the larger branches of a tree

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "bough":
   appendage, arm, bine, branch, branchedness, branchiness, burgeon,
   deadwood, flagellum, fork, frond, hand, imp, joint, leg, limb,
   link, lobe, lobule, member, offshoot, organ, pinion, ramage,
   ramification, runner, sarment, scion, shoot, slip, spear, spray,
   sprig, sprit, sprout, spur, stolon, sucker, switch, tail, tendril,
   thallus, twig, wing