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1.
The single most important thing about a breast past the A cup.

BTNR stands for "Boob To Nipple Ratio."

A girl with a large rack and small nipples is said to have a good BTNR, while one with large nipples is said to have a bad BTNR.
"Hey bro, what's the BTNR on her?"
"Sub par. The nips are like pancakes."
by Andrew Helms November 20, 2011
 
2.
<Breathing Through Nose Rapidly>

When you read/see something on your computer screen that doesnt make u laugh, but makes you breathe through your nose rapidly
Jacob: My favourite band is Nsync.

Nikki: BTNR, ur joking right?

Jacob: No? why would i, uve seen my itunes.
by HeadlessHorseman Entertainment April 21, 2011