When a woman has gone without wearing knickers when she's wearing a skirt, and she's getting drunker and drunker by the second. Skirt cleavage is the accidental nipslip of the lower half. It's usually the time when you should tell the lass to lower her skirt a little, and she takes it the wrong way.
Man: Saw a girl last night, skirt cleavage down to the knees.
by Thegreatestsongintheworld July 26, 2012