The act of or ramification of gently rubbing someone's arm to set at ease the muscle and subsiquently smashing the middle knuckle of the middle finger or pointer into the area of arm just relaxed pruducing an amusing but agonizingly painful muscle spasm resulting in an elevation of skin and muscle in the shape of a ball of various sizes that slowly makes its way down the afflicted area until fading back into the arm.
"That kid got the worst monkeyball i've everseen, it popped up atleast an inch!"
The act of jumping onto someone's chest, securing your private area near their mouth and beating the top of their head with your fists while yelling your favorite curse word.
"If you keep talking to me with no respect I will monkeyball you I swear."