capital

with a capital [A/B/C etc.]

1. something that you say in order to emphasize a particular quality. You're trouble with a capital T, you are!

2. if you talk about a subject with a capital A/B/C etc., you mean the most formal and often limited understanding of that subject. The Academy has been criticized for being too traditional and only supporting Art with a capital A.

---

with a capital letter

to an extreme degree. That makes me feel OK with a capital O. If you want culture with a capital C, you can go to an art museum or a concert.

Usage notes: used to emphasize the meaning of a particular word