gills

Found on fish and a few other animals, gills are flaps of tissue, normally on or behind the animal's head, through which dissolved gases can pass. As water passes over the gills, oxygen enters the fish's bloodstream and carbon dioxide leaves.

The word is also used to refer to structures on other things, such as mushrooms, which resemble fishes' gills.

Gills has also been used to refer to how full something might be. Stuffed to the gills would mean something was so full it was now coming out of the gills. If you stuffed a fish with a zillion shrimp, at one point the shrimp would start to ooze out of the gills.