Everyone thinks they're as funny as a comedian, even if they really
aren't. Often comes after someone makes a
joke that was only funny to them, timed too late, or overly
snarky.
Dave: "Man, it's really
hot out today."
Jack: "Tell me something I don't know, Captain Obvious.
Maybe it's because the giant ball of fire in the sky is out?"
Dave: "Alright, everyone's a comedian."