Punt
From Assassin Wiki
A player decides he isn't having fun or an emergency (or not-so-emergency) comes up, so he stops coming to game: this is called a punt. It can also happen pre-game, where a player who had been cast is unable to play (this is why it's a good idea to maintain several players on your waitlist, for this inevitability.) Mid-game punts suck: it screws over their team-mates, unbalances the game, and is generally a douchebaggy thing to do.
However, the problem of players not having fun is a very real one, and it is why longer games have characters serving as roles as Game Mommy, to help players over rough patches. Not everyone can win, but everyone can have fun.