Rings of connective tissue whose elasticity and semitransparency belie their strength.
I'm not talkin' about the cartilaginous rings that hold open tracheas. I'm talkin' about the cartilaginous rings that hold together six packs. Or more—unlike "six pack rings", "beer cartilage" doesn't discriminate among multi-packs.
(Beer cartilage is an item in Kingdom of Loathing. As far as I can tell, the term originates there and isn't used outside the player base—a shame, as it's a nice term.)