“What do you have on tap?”

You're at a restaurant. The waitress asks what you want to drink. You'd like a beer, so you ask this.

What do you have on tap?

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have (a brand of beer) on tap

A lot of restaurants and bars have beers "on tap", meaning that they have big kegs of beer that they can pour glasses from. To get the beer out, they "tap" the keg, which means that they attach a special beer-dispensing hose to it.

If you want to drink a beer from a glass or mug instead of from a bottle or can, you can ask the server or bartender what they have "on tap".