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"