The quiet, silent, gal in your office who has more miles on her than Liz Taylor's mattress with questionable hygienic practices. Oft times you can see the fluid building up in her shoes.
A Dirty Leg is a slutty redneck type girl. AKA A Dirty Bird. Usually found drunk and smoking a cig. Term probably comes from the assumption that something is dripping down her leg.
Where'd you find that Dirty-Leg you were with last nite? I picked her up at Coyote's.
A girl that is such a slut (and has so many different guy's cum running down her leg) that (i) she either lost her shoe during all the fucking that she is doing or (ii) can't afford a second shoe.