(n.) Nickname of the seed pods produced by sugar maple (Acer saccharum) trees (Liquidambar styraciflua, according to other sources). When ripe, these hard, spiky green balls are about the size of ping pong balls, and make excellent, abundant projectile ammo.
"Okay- everybody grab your sling shots and rackets, and pile up the monkey balls! THE WAR IS ON!!!"
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 ever seen, it popped up atleast an inch!"