Sweetbread is the culinary name for the innards of calf, lamb or even beer and pork. It is also called offal. Those who don’t like it call if awful. It is typically the thymus or pancreas.
The word dates back to the 16th century. The word “sweet” probably omes from the sweet tasting thymus and “bread” from brede which means roasted meat.
While there are many preparations, the common one is soaking sweetbread in salt water, the outer membrane removed, and then poaching it in milk. While this may not sound appetising, it is then dried, chilled, breaded and fried. And as you know, everything tastes good when fried. This can used to stuff pates, grilled in Latin American cuisine or served with break in Turkey.