Bumrush

[Tanya Shivari]

(n) Crash the gates of a show with a pack of people, in the hope to get in without having to pay. Yo! Bumrush the show -- Public Enemy. (Yo! Bumrush the show) [1987].
(v) To get beaten up by a large, angry group of people. Literally to get rushed by bums or lowlifes.

[Dave Paprocki]

The sudden act of [anal penetration] without the victims awareness.

[Luis Fok]

To be making sex in the vagina and accidentally slip out and thrust into said bum

[Jessica]

The act of [anal sex] between two males, by the person [standing firmly behind]. While it can theoretically also refer to [heterosexual] anal sex, bumrush is exclusively used in practice to refer to anal sex between two men. More specifically the [tops] does the bumrushing, while the [bottoms] gets bumrushed.