To set out or depart from some place: go. Most likely used in a party setting where any number of factors may cause you to want to leave (such as a sausage fest, lack of beers, people who aren't as cool as you are, etc.).
Camera voyeurgasm of sticking one's mob. down-up an unnoticing girl's skirt in subway cars and *click* *click* on the booty!
Up-skirting is the down-up version of down-blousing, where the pix are taken from above tits to capture some breastcleavage or boorak, assnecks, canal zone!