An attempt to mock someone for being angry or annoyed, often used by trolls or just dickheads in a cheap attempt to make their target more angry.
This response is usually predicated on the questioner's arrogant notion that they themselves are immune to losing their cool and never get angry when someone is being a dickhead to them.
This usually has 0.000001% likelihood of being true.
Possible response: "Yes, I mad, and if you weren't hiding behind an anonymous identity on the Internet, I'd find you and let you know just how mad I am."
Person 1: "Stop fucking micspamming and hacking you fucking dickcheese!"
Person 2: "Lol u mad?"
Person 1: "..."
(a few days later)
Person 2: " YOU KILLED MY FAMILY AND BURNED MY HOUSE DOWN! YOU BITCH!"
Person 1: "lol u mad?"
Asking someone if they are angry.
Jerry: Did you topdeck that graceful charity into two goldds and a CS to attack into my reaper for game?
Random Scrub: U mad?
Sammy/nick/ygo community: Why are you so much better than me, michael?
Michael: U mad?
Taunting others in a way that's hilarious. If anyone is actually stupid enough to fall for this the real joke isn't just the fact that they get angry, it's that they actually fall for something so overused and STILL take the bait. That is truly the funniest form of stupidity.
Person A: Shut up, and stop saying that.
Person B: U mad?
Person A (Moron): Fuck you.