(You don't need the ONyomi because the only useful jukugo is KUNKUN style)
Yo Mama's mouth is so filthy, she has to get a rock from the cave to chop down a tree in a grove to make an adequately sized tooth - brush.
brush :(almost always used with teeth: 歯を磨く （は を みがく)
the act of brushing the teeth ('Brush your teeth' = 歯を磨きます!）
Your HAND makes friction, as you brush ROCKs looking for gold.