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."
Used as an exaggeration for a time period between something that just happened and the last time that same thing happened.
Matt: I'll beat you at ping pong again, like I did last time.
Caleb: Matt the last time you beat me at ping pong was back when Britney Spears was still hot.