Definitions of pestle:

• noun:   a club-shaped hand tool for grinding and mixing substances in a mortar
• noun:   a heavy tool of stone or iron (usually with a flat base and a handle) that is used to grind and mix material (as grain or drugs or pigments) against a slab of stone
• noun:   machine consisting of a heavy bar that moves vertically for pounding or crushing ores
• verb:   grind or pulverize in a pestle