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 —
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.