When the skin at a womans arm pit folds to resemble a pussy. Arm pussy, often abreviated to "AP" is the ultimate form of voyuerism. Firstly, women don't hide this part of their body in public, but more importantly AP is a very close representation to a womans actual vagina in terms of lip tightness, and hairyness. AP is a public preview of private nudity.
Christ, check out the the arm pussy on that chick at the next table.