1) A time during the day wherein an inconvenient event occurs at the most inopportune moment
2) The point in time at work where you're about to close when something happens that sets your work back a considerable amount.
Person 1: "Dude, did you hear the news?"
Person 2: "No..."
Person 1: "We close in less than half an hour and that group of 30 that canceled their reservation have decided to show up, so they're probably not going to order for another 20 minutes."
Person 2: "Yeah... You can definitely tell its asshole o'clock"