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Available for immediate use, ready, as in We have two more trumpeters on tap for the parade. This metaphoric expression alludes to a beverage such as beer that is ready to be drawn from a cask. [Mid1800s]
American Heritage Idioms
2
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Phrase(s): on tap
1. Lit. having to do with beer served from a barrel or keg. • Do you have any imported beers on tap here? • I like beer on tap. The canned stuff tastes funny to me. 2. Fig. immediately available. • I have just the kind of person you’re talking about on tap. • The cook has any kind of food you might want on tap.
McGrawhill's American Idioms And Phrasal Verbs