Sweetbread is an edible part of an animal, specifically...
Sweetbread is a culinary name for the thymus (also called throat, gullet, or neck sweetbread) or the pancreas (also called heart, stomach, or belly sweetbread), especially of calf (ris de veau) and lamb (ris d'agneau), and, less commonly, of beef and pork.1 The "heart" sweetbreads are more spherical in shape, while the "throat" sweetbreads are more cylindrical in shape.
Will is already starting to think like the killer and has already deduced that the killer is eating human body parts...he says to Lecter
"Then suddenly I had a flash of the third victim...
She was missing flesh from her back. And then it hit me.
Liver, kidney, tongue, thymus.
Every single victim lost some body part used in cooking."
AND he finds it odd that Lecter hasn't come to the same conclusion...
"You're the best forensic psychiatrist I know... and somehow, in all our time together... this possibility never occurred to you."
Then on perusing Lecter's collection of rare books, he comes across a French cooking book in which, apparently, Lector has written the word "sweetbreads".
Basically, at that point, he puts it all together.