The act of exiting one side of a moving
automobile (usually through the
window, as a door catches the wind and can slam on you), climbing over the
bonnet or
boot (or the roof, if you're confident) and re-entering the vehicle on the other side. Usually this is done without any
safety equipment, but if the car is also equipped for
car surfing, there are usually ropes present. Don't decide to be clever and press your face against the
windshield, as the
driver may turn the
windscreen wipers on, and those things actually hurt a bit. Typically,
alcohol helps to convince people that getting out of a moving
car and getting back in on the opposite side is a
good idea, but it can also be a useful way to change seats without stopping.
Also (originally) the term used to describe an
astronaut (or
cosmonaut) exiting their
spacecraft and traversing the outside of it. They can be
tethered (attached with a string type affair, only more hi-tech) or untethered, and they may or may not have some sort of
rocket pack type affair.