Something tapped at a bar

KEG

Think of the large metal container that holds beer before it’s poured into your glass.

This clue plays on the double meaning of "tapped." At a bar, a keg is literally tapped to release beer, but "tapped" can also mean something being lightly struck or knocked. So while you might picture someone knocking on a bar counter, the answer is actually the beer container itself—something that gets tapped in both senses of the word.