with a capital A, B, C, etc.

with a capital (some letter)

1. In the most extreme form or degree. I am hungry with a capital H! Let's eat! No, it's not an emergency with a capital E. I can wait until the end of the day.
2. In the most typical, formal, or traditional form. When he talks about photography, he means with a capital P. He would never think to consider pictures taken on smartphones. Well, it's not literature with a capital L, but it's still a good story.
See also: capital
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

with a capital ˈA, ˈB, ˈC, etc.

used to emphasize that a word has a stronger meaning in a particular situation; very: When I say he’s boring, I mean boring with a capital B!
See also: capital
Farlex Partner Idioms Dictionary © Farlex 2017
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