To overbook means to accept more reservations or commitments than you can actually handle. Airlines overbook flights by selling more tickets than there are seats on the plane, betting that some passengers won't show up. Hotels overbook rooms for the same reason: they know from experience that several guests will cancel at the last minute.

The strategy usually works, but when everyone actually shows up, there's a problem. An overbooked flight might leave passengers stranded at the airport, waiting hours for the next available flight. An overbooked hotel has to scramble to find rooms for guests at other hotels nearby.

You can overbook yourself too. If you promise to help three different friends with their projects on the same Saturday afternoon, you've overbooked your time. You meant well, but now you can't keep all your promises. Students sometimes overbook their schedules by signing up for too many activities, leaving no time for homework or rest.

The word suggests a miscalculation: someone counted on things working out perfectly, but they didn't leave room for the unexpected.