The immediate feeling you get when you just put a large amount of time of work into something, something that you crafted yourself, something that is special to you... only to have all of it suddenly squandered away at the very end.
Named after the game Mass Effect 3, which arguably has the worst ending in video game history.
I once had a puppy I bought and nurtured all by myself. He was growing into a healthy dog; he was very happy and we had so much fun. Then some horrible person decided to kill him and he got away with it. I Mass Effect 3'd for the rest of the year.