A covering over a liquid (see also film). ("In order to get to the rest of the paint in the can, you'll have to remove the skin floating on top of it.")
A very small amount of margin for error, or small likelihood of success. ("We'll probably just make it by the skin of our teeth.")
A pornographic or adult movie. ("It's a skin flick.")
A replacement image for the graphical user interface of a computer program. ("You can use this skin to change how the browser looks.")