Gas her back end

To fuck one's back end

Bro 1: Bro I wanna hit up this thottie but I don't know what to say
Bro 2: Bro just tell her you're gonna drug her and gas her back end.

Smash your back end out

To have hard, vigorous sex with a person.

John: "I'm gonna smash your back end out."

Gas her back end

To fuck one's back end.

Bro 1: Bro I wanna talk to this girl but I'm not sure what to say to her.
Bro 2: Bro just tell her you wanna gas her back end.

Las' back-end

Last winter.
Used in northern England (mostly in Yorkshire).

wu'd yall las' back-end ?
what where you doing last winter