Ride shotgun

To ride shotgun means to sit in the front passenger seat of a vehicle during a trip. Figuratively, the phrase refers to the support or aid given to someone in a situation or project.

The expression riding shotgun is derived from shotgun messenger, a colloquial term for express messenger. A special armed employee of the express service using the stage for transportation of bullion or cash would sit beside the driver, carrying a short shotgun (or alternatively a rifle), to provide an armed response in case of threat to the cargo.

Example: Would you like to come to the party and ride shotgun? We really need your help!