A hacker. Usually used by noobs and hackers, but can also by used by other people. Mostly used on Roblox, where game hacking is common. A noob will often say "H4X!!!1112" if they see a hacker in action.
Noob: OMG H4X0R!!! U H4X MAH UB3R G4M3!!!! H4X! H4X!!!1
When your thumb gets all red and sore from pressing the D-pad on a DS or other video game console for too long.
I was playing Mario 64 DS for 4 hours straight, so I got Red Thumb Syndrome.