means A smoothbore firearm that fires a spray of small pellets, scattering its shot over an area rather than firing a single aimed bullet; also, by extension, the front passenger seat of a car.

from A 19th-century American coinage, simply 'shot' (the lead pellets) plus 'gun.' The front-seat sense comes much later, from the 'shotgun messenger' who rode beside the driver of a stagecoach in the American West, cradling a shotgun to fend off robbers — so calling 'shotgun' for the passenger seat is, quite literally, claiming the gunman's perch.