A phrase used often by greats and salarymen to express stress, frustration, distress, or any other negative emotion that comes to mind. But it may also be used to express excitement as well. Often muttered quietly to one's self, but may also be said loud and publicly when the mood is right.
Shinra Manager: "Hoo boy....hoodlums again. God, don't I just have all the luck..."
Dio: "Hoo boy!"