Western Canadian expression to describe an ugly female that hunts for men outside of bars and strip clubs. Normally they hang around the roadside or parking lot at the end of the evening hoping to get picked up by the desparate and dateless. Hence the 'Ditch' part of the name. 'Pig' is self evident.
Most ditch pigs end up becoming coyote dates the next morning.
Kathy is such a ditch pig, she blew 4 guys in their pickup outside the French Maid.