The girl you call when you can only get to first base, because she will run the rest of the bases for you. aka give you sex when you cant pull the dime.
i was hooking up with this dime last night but she wouldn't put out so i had to call my pinch runner.
my lead off batter (main slampiece) fell though tonight so i called in the pinch runner for the late night booty call.
When you are unable to complete a task, and need someone to come in and assist you.
person 1: "Hey man, did you see Kleim last night? Kid totally couldn't handle his alcohol."
person 2: "Yea dude, he needed a pinch runner"