I had never heard of sweetbreads before I began reading old cookbooks. Sweetbreads are what the thymus gland or pancreas of a calf or lamb are called.
Eating offal or organ meats (the parts of the animal that are not muscle) is becoming popular again as shown by this cookbook:
Odd Bits: How to Cook the Rest of the Animal
RECIPES BELOW FROM OLD COOKBOOKS
SWEETBREADS WITH MUSHROOMS
Lay half a dozen sweetbreads in cold water for twelve hours, changing the water several times. Then boil them five minutes, drop into cold water, remove the skin and lard* with fat bacon. Put them in a saucepan with a pint of stock, two small onions and one carrot chopped, a teaspoonful of minced parsley, salt, pepper, cayenne, and a little mace. Stew until tender.
Serve with a mushroom sauce, made as follows: