The correct definition of the word is actually:

Bumrush orginated when homeless peoples would come into a store and start loitering and casuing disturbances looking for places to sleep/eat/take shelter. These are stores such as super markets or retail stores. Store managers, became disgusted and tired of the homeless and degenerate roaming through their stores in search of warmth and possible indulgence of a twinkie or two. They went into the back storage room and usually took a broom and then went on a frenzying rush to rid their stores of bums. Thus the store went back to it's original status of a respectable and pleasant place to shop.

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.

The sudden act of anal penetration without the victim's awareness.

1) (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.
2) (v) To get beaten up by a large, angry group of people. Literally to get rushed by bums or lowlifes.

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

What happens to a street punk and gansta wannabe in the shower on his first day in prison.

1.(v) To blindly charge at
2.(v) to attack in force, to use in force
3.(v) to raid (As in pillage/common tatic in online games)

To attack someone or something by wildly charging them full force with no sort of tact or skill other than one's own momentum and possibly the backs and sides of their fists as they randomly flail them out of rage, but certainly consisting of no precisely swung blows. Also bum rush.