Similar to pregaming, drinking between two social events (or sometimes midway through one event) usually at someones house. Like pregaming, it can be to avoid expensive drinks in town or to just be drunk on reaching the second event.
We're going out for dinner then back to Dave's to midgame at his flat before going to a club
If you're in the process of playing a video game and you are called by your parents to do something, shout the universal term, I'm Mid Game. Your parents will understand that you cannot be bothered at this moment and you will see them when the game is over.
A. "Hey Bren, come downstairs for dinner."
"But Dad I'm Mid Game."
B. "Yo can you send me the homework on aim?"
"Sorry bro, not right now, I'm midgame. In a few."