Someone who is extremely stupid. They will often lack even the basic knowledge common to small children. Derived from the derogatory phrase "the best part of you ran down your mother's leg."
Person 1: "Yo, that guy just asked me what part of Africa Canada was in!"
Person 2: "Dude's a total Leg Runner..."
Person 2: "Dude's a total Leg Runner..."
by HarbingerOfDumb May 9, 2018