What does C-Cup mean?
A reference to the bra cup size. While some may believe this is the average or "best", two complications come to mind: 1) the average is hard to determine for various reasons, as it has increased over time, some women are wearing incorrect sizes, and different brands size differently, etc. And 2) this is all subjective. Some people like smaller, some larger.
Other definitions of C-Cup:
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How to use the term
C-Cup:
When I hit puberty, I went straight from an A cup to a C cup. Now I'm somewhere around DD, and my bra fits much better. It pays to do your research, and my back loves me for it.
References:
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More slang terms:

YQW
You're Quite Welcome

TC
That's cool

YFW
Your Face When ...

4GET
Forget