The Collaborative International Dictionary of English v.0.48:

Udder \Ud"der\, n. [OE. uddir, AS. [=u]der; akin to D. uijer, G.
   euter, OHG. [=u]tar, [=u]tiro, Icel. j[=u]gr, Sw. jufver,
   jur, Dan. yver, L. uber, Gr. o"y^qar, Skr. [=u]dhar.
   [root]216. Cf. Exuberant.]
   1. (Anat.) The glandular organ in which milk is secreted and
      stored; -- popularly called the bag in cows and other
      quadrupeds. See Mamma.
      [1913 Webster]

            A lioness, with udders all drawn dry. --Shak.
      [1913 Webster]

   2. One of the breasts of a woman. [R.]
      [1913 Webster]

            Yon Juno of majestic size,
            With cowlike udders, and with oxlike eyes. --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

udder
    n 1: mammary gland of bovids (cows and sheep and goats) [syn:
         udder, bag]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "udder":
   bag, boobs, bosom, breast, breasts, brisket, bust, chest, crop,
   dug, knockers, mama, mamelon, mamelonation, mammary gland,
   mammilla, mammillation, nenes, nipple, pap, papilla, pigeon breast,
   teat, thorax, tit, tits, titties, titty