Certain arm muscles, for short

TRIS

Think of the three-headed muscle that flexes when you show off your strength.

The clue refers to the triceps, the muscles on the back of your upper arm responsible for extending the elbow. "For short" signals that the answer is an abbreviation, so "triceps" gets shortened to "TRIS." It’s a common shorthand in fitness or anatomy contexts, making the answer straightforward once you recognize the term.