means A heavy, rounded tool used to crush, grind, or pound substances into powder or paste, typically against the inside of a bowl-shaped mortar.

from From Old French 'pestel', drawn from Latin 'pistillum', a little pounder, which traces back to the verb 'pinsere', to pound or crush. That same crushing root is a cousin of 'piston', which still slams up and down for a living, and lurks behind 'pistil', the flower part the early botanists thought resembled a tiny pestle standing in its bowl.