Top Definition
The rare undercroft is a spectacle only seen to the luckiest of man. This wonder is only reported to be viewed 27 times in recorded human history. The undercroft is the breast and under cleavage shown by a revealing bikini, brassiere, or tank top. It is usually seen by mistake and can be embarrassing to the woman with it. A more clear definition is, "cleavage visible from the part of the shirt that covers the bottom of the female breasts".
Hey bro, did you see the undercroft on that chick?

No way bro I've heard that only occurs once in a blue moon!!

I know bro! I just exploded in my pants.... yikes
by Charlie Devoe January 12, 2010

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