Game over, man!

Usually a screen after a game is over due to loss of all lives or failing a critical objective, especially in older (video) games.

It is conceding defeat, in this instance. Bill Paxton's Private Hudson famously says this line in ALIENS, after the drop ship coming to rescue him and his expedition crew crashes.

Private Hudson : (after the drop ship crash) Well, that's great. That's just fuckin' great, man! Now what the fuck are we supposed to do? We're in some real pretty shit now, man!

Corporal Hicks : (grabs him by the shirt) Are you finished?

Newt : I guess we're not gonna be leaving now, right?

Ripley : I'm sorry, Newt.

Newt : You don't have to be sorry. It wasn't your fault.

Private Hudson : That's it, man. Game over, man! Game over! What the fuck are we gonna do now? What are we gonna do?