under clevage 

when a girls boobs are showing from the bottom of her bikini

damn that chick's got major under clevage hanging

under clevage by Rebecca 757 June 19, 2007

undercleavage

Cleavage that can be seen from beneath a woman's top (or the strip of fabric that passes for it).

Undercleavage is nice and all, but you almost have to wonder how her boobs don't slip out of her top...

undercleavage by nat s. October 16, 2008

undercleavage

(n.) - The bottom parts of a woman's breasts below the nipples that are visible the bottom of a very short midriff shirt or t-shirt that covers the top of her breasts.

"If that's not a deliberate display of undercleavage on that model in the Vogue photo, I don't know what is!"

-- overheard at a watercooler in Los Angeles

undercleavage by PRwiz101 October 17, 2009