A knockbox, is a device used to store spent espresso

Espresso

Espresso is a concentrated beverage brewed by forcing a small amount of nearly boiling water under pressure through finely ground coffee. Espresso is widely known throughout the world....

 grounds, called a puck, after a shot of espresso has been pulled. It is generally made out of stainless steel or plastic, and has a sturdy bar known as a "bash bar", against which a portafilter

Portafilter

A portafilter attaches to the group head of semi-automatic and piston-driven espresso machines, and carries a tamped puck of coffee grounds within its basket. It is usually made of brass, and is attached by a plastic or wooden handle...

is tapped to release the puck. Because the espresso puck is typically very hot after extraction, knockboxes need to be sturdy. Further, they are also subject to repetitive knocking and tapping to dislodge espresso after use, and thus rubber and stainless steel are the most commonly used materials. In various coffee houses they are seen as drawers which are integral within the bar structure, or as stainless steel boxes usually located near the grinder for convenience.