It happened to me today. I was in the one and only squat rack at the gym (squatting, of course), when a 30-something guy in a sleveless t-shirt came up to me between sets.
"How many sets do you have left?"
"You mean you have 3 sets left to go?"
Yeah, you can work in if you'd like.
"Nah, thanks, I'm doing arms."
As he wandered off in search of a more bicep-friendly environment, I was left shaking my head. Besides the fact that this guy wanted to do bicep curls in the squat rack (don't get me started), the phrase "I'm doing arms" just annoys the dickens out of me.
Training only your arms is pointless. In the real world, your arms never work in isolation. Training movement patterns, not body parts, is the way to get stronger.
Instead of "arms," train pulls: pullups, chinups, rows. Instead of "chest," train pushes: bench press, overhead press, pushups, dips. Instead of "legs," train... well... legs, but use functional compound exercises: squats, deadlifts, cleans.
The phrase "I'm doing arms" aggrivates me because it reflects a fundamental misunderstanding of this basic concept of strength training. To get strong, you must move heavy things in a way that closely mimics the movements you'd use to move heavy things in real-life scenarios outside of the gym.
Maybe I should be more understanding. Maybe I should try to offer a tidbit of friendly advice next time this happens. Or maybe I'm just a crotchety old man in training. But at least I'll be a crotchety old man capable of physically throwing those young bicep-curling whippersnappers off of my lawn instead of meekly shaking my cane at them.