Chest muscles, for short

PECS

["Think of what bodybuilders often flex to show off their upper body strength—this term is also slang for a certain type of informal conversation."]

This clue is asking for the shortened term for "pectoral muscles," which are the large muscles in the chest. "Pecs" is a common informal abbreviation, like calling biceps "bis" or abs "abs." The "for short" in the clue signals that the answer is a nickname or shorthand version of the full term.