New word: Escapegoat

n. one who serves as the excuse to leave a party or other gathering earlier than generally socially acceptable, i.e. "Oh, I'm sorry, but Dave needs to get home to his sick mother," "We'd stay, but Helen is pregnant," or "Richard hates it when people call him Dick. Now we have to go before he gets dangerously drunk and vomits on your bathroom floor...again. We had a great time."