When you're in a public transit, like a bus or train or plane, you pull out your penis before the person next to you sits down and you put it under them with out them knowing.
Marc: Hey Nathan, do you remember that time i lumber jacked that chick on the bus?
to jack someones lumber or to be jackin' lumber lumber is basically to give someone a hand job.
Guy:"wanna come over tonight and jack my lumber?"
Girl:"eww, thats disgusting I'm never even going to touch you perv!"
Guy:"oh come on, its not like you've never been jackin' lumber before"
Girl:"Fuck You!"