Midgame
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
Midgame
To retreat to one's car or room at half time to drink more during the game or other event.
This lacrosse game is such a bust, we will probably need to midgame in order to deal with it.