when you got to a concert expecting a certain type of music and end up watching a jam band.
Dude, we went to go see System Of A Down and the opening band sounded like Phish. We got jambushed.
When you are playing music at an open mic night and a complete stranger jumps on stage and starts playing with you without asking.
Highly annoying, especially if said stranger isn't nearly as good as they think they are.
A portmanteau word from "jam" and "ambush." Past tense "jambushed."
Roommate: Yo, how was open mic tonight?
Bleeding-heart songwriter: Lame. Got jambushed by this drunk harmonica player. Just leaped up without saying anything and started doing terrible, terrible things to my song.
Roommate: Sucks, dude.
When someone walks into a room full of jewish people and doesn't fit.
"Dont go in there cathy, its a Jambush.".
"Thanks dave, i owe you one"