thymus

noun

Etymology: From New Latin thymus, from Ancient Greek θύμος (thúmos, “warty excrescence, (also, as used by Galen) thymus gland”).

1. Ellipsis of thymus gland (“A ductless gland, consisting mainly of lymphatic tissue, located behind the top of the breastbone. It is most active during puberty, after which it shrinks in size. It plays an important role in the development of the immune system and produces lymphocytes.”).

   “For a very long time the role of the thymus in the body was a complete mystery.”