muddler
A muddler is a bartender's tool, used like a pestle to mash — or muddle — fruits, herbs, and/or spices in the bottom of a glass to release their flavor.
Four popular cocktails that require the use of a muddler are the Old-Fashioned made with whiskey, the mojito made with light rum, the caipirinha made with cachaça (as well as the caipiroska made with vodka instead of cachaça) and the mint julep made with Bourbon whiskey.