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1.
Earning the right to ride in the front passenger seat of someone's car. This is also the direct opposite of "riding bitch," where you are sitting center-backseat, sandwiched by your friends (aka: riding gay, riding whore). It is "shotgun"'s responsibility for controlling car temperature, changing radio stations, calling out landmarks, and flagging down hot girls.

Variations to "calling shotgun" can include paper/rock/scisors, coin flip, or being the first to yell "shotgun" when the driver pulls their car keys out of their pocket. Seniority rarely means anything when calling shotgun.
Steve was riding shotgun because he won the coin flip in the mall.
by D Bizz May 31, 2006
 
2.
Sitting in the passenger's seat
by Anonymous February 24, 2003
 
3.
Where you hang out a window doing a drive-by
In Grand Theft Auto San Andreas you can be Riding Shotgun
by Ballerz74 October 24, 2007
 
4.
To blow your brains out or contemplate the same.
I'm feeling a lot better. I'm no longer riding shotgun.
by Whiskey Peter December 08, 2007