The Collaborative International Dictionary of English v.0.48:

Sweetbread \Sweet"bread`\ (sw[=e]t"br[e^]d`), n.
   1. Either the thymus gland or the pancreas, the former being
      called neck sweetbread or throat sweetbread, the
      latter belly sweetbread. The sweetbreads of ruminants,
      esp. of the calf, are highly esteemed as food. See
      Pancreas, and Thymus.
      [1913 Webster]

   2. (Anat.) The pancreas.
      [1913 Webster]

WordNet (r) 3.0 (2006):

neck sweetbread
    n 1: edible thymus gland of an animal [syn: neck sweetbread,
         throat sweetbread]