to hit something or someone with a closed fist in an overhand motion intending to cause damage, similar to the way a hammer is swung using the tricep muscle. Usually quick and repetitive.
Brock Lesnar landed some mean hammerfists against Frank Mir.
"Quit being stupid before I knock you down and hammerfist your face!"
"Quit being stupid before I knock you down and hammerfist your face!"
by Forrest Griffin January 16, 2010