When someone unexpectedly walks in or when you drop a towel in a changing room and sees your Bush in all its glory.
"Jessica's brother walked in when I was using the bathroom at her house. I was mid pee and jumped up, he amBushed me. I should have shaved."
The act of ambushing your friends with a chick on your arm at an clearly understood testosterone-only event, such as a poker party or chasing skirts at a club - esentially, bringing sand to the beach.
I can't believe John ambushed the poke party with his girlfriend. Usually, I take his money before showing him the door, but in this case I didn't even bother opening it.