When someone wraps their legs around another, locking them between their legs and body. This term is mainly used in a sexual sense.

When a woman wraps her legs around a guys neck and puts him in a chokehold while he eats her out.

when you’re desperate/fiending for pussy and are trapped in a females grasp

While recieving a deepthroat from a bitch, one proceeds to wrap his legs trapping the girl in the deepest part of a deepthroat. Then, the male puts a clothes pin on her nose ensuring that she cannot breath. It is important to keep a straight face the whole time, and if a camera is involved, stare directly into the camera.

A leg lock is a slow painful death to the running man. You're the runner, one leg is forward and the other is back there somewhere and you've just got to get away but before you get a chance that guy/gal slides on the ground underneath you and figure fours their leg around your straight back leg. You flip around to kick it off as their legs are closing in on yours and snap.. you don't know if it's broken, you don't know if it's out of it's socket. You're gimpy. You should probably see the doctor but you're afraid you'll look like a pussy. It's okay bruh, but seriously if it's not better in three days go see a doctor.

When he fucks you so good your legs lock up.

One of the most painful submission moves in wrestling today. It became popular due to the legendary Nature Boy, Ric Flair using it as his signature move. As such, a wrestling crowd will start shouting "Wooooo!" as a tribute to the now retired wrestler. Example linked below.

http://www.youtube.com/watch?v=i4YD4LVpk3E