The act of sticking a piece of rock into a clump of grass, then tying it together. It's then used to throw at a person you dislike.
It's like stuffing a rock in snowball to make it hurt more.
Steve: "I hate Samantha. I wish I could injure her, but grass isn't dangerous.
Phil: "Here, make a grass bomb. Stick a rock in it, it'l hurt more.