monkey balls
(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.
Example:
"Okay- everybody grab your sling shots and rackets, and pile up the monkey balls! THE WAR IS ON!!!"
"Okay- everybody grab your sling shots and rackets, and pile up the monkey balls! THE WAR IS ON!!!"