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1.
When your nipples get hard.
It's cold in here, I just got a nipple boner!
by flash dance hobo May 12, 2006
 
2.
When your nipples get hard/erect

alternative to a man having a erect penis but, this occurs in a different part of the body.

may happen whether you're male or female. though I like personally calling them nippleboners on the ladies or preferably myself.
"Wow look at Mandi wearing that wet tshirt you can totally see her nippleboners!"

"Mandi always said whenever she woke up from sleeping she'd awake with nippleboners"
by Mandi Marie Elgo October 16, 2005
 
3.
When your nipples become hard and stick out.
Tom "He i can see her nipples through her shirt."

Jim "Ya, she has a nipple boner."
by jimbo4444 July 24, 2012
 
4.
A nipple boner is usually a small foreign penile protruding from a woman's breast; Usually milky in color and it smells like shat.
Ron: Is that a little penis coming out of your nipple?

Black girl: No dick, it's my nipple boner.
by Oprah Adolf Hitler April 03, 2009