It was a group of female supporters of Hillary that were irate that Obama knocked her out in the primaries and they pledged their undying efforts to make him pay for it politically. Then he got elected anyway, and the PUMAs probably voted for him in the end.
I don't remember what it stands for.
I believe it stands for Party Unity My Ass.
PUMAs were irate not because Obama knocked her out in the primaries but that the superdelegates played games and handed him the win; he hadn’t earned it.