Bone Cracker

The Bone Cracker (or Beat Down in Halo 3) Medal is awarded for hitting and killing an opponent with a melee attack that isn't an actual assassination (melee attack from behind). It is available in all Multiplayer gametypes and has a Bungie difficulty rating of 3 (out of 10). It is represented by an inverted silver star with a dark green fist. After you kill an opponent with a melee attack, a message appears on your screen that says: "You beat down (player's name)". It is also awarded if a Covenant Energy Sword or Gravity Hammer is used to kill an opponent from the front.

The Green Fist
The "Green Fist" could be a reference to the Incredible Hulk, the Hulk using it's fists a lot and the fact they're both green.