When you are driving and need both of your hands for something, the passenger can be asked to take the wheel while you do what you need to do. Can be used as a noun or verb.
Friend 1: "Hey, do you want a hit this bowl?"
Friend 2: "Sure, can you copilot for a sec so I can light it?"
"I thought I was gonna get pulled over because I was swerving all over the road, but my buddy is just a bad copilot."