以下内容摘自四部英文词典：

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.

with a capital A/B/C, etc.

1. You say with a capital A/B/C, etc. to mean that something has a particular quality to a great extent. You mark my words, that man's Trouble with a capital `T'.

2. You say with a capital A/B/C, etc. to mean that a particular idea or concept is being understood in only the strictest sense. The British tend to see things in terms of principles with a capital P. This is art with a capital A. Note: This sense is often used slightly disapprovingly, to suggest that someone is taking something too seriously.

Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

with a capital — used to give emphasis to the word or concept in question.

1991 Nesta Wyn Ellis John Major He is not a personality with a capital P, not flamboyant, not it seems an angry man.

Farlex Partner Idioms Dictionary © Farlex 2017

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! 

我说他很无聊时,意思是他简直无聊透顶!