Penis, but usually used with "Riding" when you are having to bend over and take it - like an impossible and painful situation you can't get out of.
My boss has me riding the skin bus on this project.
to masterbate at great speed
'I woke up this morning and took a white knuckle ride on the skin bus'
Penetrative hetrosexual sex.
Man, Tonights going to be be great. I am going to drive the pig skin bus to Tuna Town with Angela.