Coined by comedian Christopher Titus, first date arm occurs when a man is taking a girl on a first date. While his arm is on the armrest he will flex it as hard as he can. That way, just in case she brushes up against him, she will think "Oh my god, he's spun steel". The resulting tired arm is called, first date arm.
"Yo bro how'd that date go?"
"It was good man, but I got first date arm really bad."