This is when a character (perhaps out of anger) punches or otherwise physically assaults an object that should cause bruised knuckles and pain, but which doesn't even cause the character to flinch. May be caused by Rule of Awesome or Rule of Funny.
Type 1: The character is so outraged that they punch an object and don't seem to receive pain.
Type 2: The character DOES receive an injury, but is too dumb to react (a la Dumb Muscle 
), or at least undergoes a Major Injury Underreaction 
Type 3: The character is in a state where they can not feel pain, and decides to abuse it.
Karate Joe in Rhythm Tengoku, Rhythm Heaven, and Rhythm Heaven Fever can punch rocks and barrels without flinching (Of course, this is only if you do it correctly).
The Heavy from Team Fortress Two has fists as his default melee weapon. He can punch anything in the game and never receive any pain (Type 2).
Stan from American Dad!, in the episode "Threat Levels" headbutts a wall after being told he's overreacting and doesn't receive an injury.
In the episode of "Dilbert", "The Infomercial", the Pointy-Haired boss receives head trauma and subverts this trope; he notices his injury, but just ignores it (type 2).
PHB: Losing... Consciousness... and yet i'm still aroused.