A special gathering or sleepover that takes place in a building from dawn 'till dusk, or 12 hours. At the beginning you're doing a bunch of activities and games, watching the movies the chaperones provide, until usually 9 hours in. And at this time everyone becomes either testy, zombie like, hyper, or bored. Usually the activities cease (except for the video game tournaments and the movies) and everyone falls asleep or just talks about a boring subject. By this time the food is all gone, so everyone is scavenging for any scrap of sugar.
The lock-in started at 7 pm and ended at 7 am. I think I passed out at 4 am.