A foot without a sock is a man who is constantly looking to have sex with someone. In other words, a man-slut. It is called being a foot without a sock becuase the man always needs something to stick himself in.
Dan: Man, it's weird that Steve has had sex with the hottest girl on the planet, and now hes going after
Sharon, and shes like a 4 out of 10!
Bob: Dude, hes just a foot without a sock.