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When a boob is slightly under water (most bathtub scenes) and you can see a slightly discolored spot in the water where a nipple is hiding underneath the surface. Kind of like a cave dwelling spider, you know it's there but you can't quite see it.
Blayne-"Chase rewind I think I just saw Amy Smart's water nipple!"

Chase-"Holy shit that is her water nipple!, man I love "Mirrors."

by Zombie Jester February 16, 2009
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