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1.
a group gathering where you are "locked" in a building. It may include games, movies, but never sleeping! sleeping is for losers :oP
Lauren and Monica went to the lock-in and didn't sleep. They were up till 6am
by Monica Jo June 08, 2007
 
2.
The largely illegal state of a bar or pub during the period of time after last call, during which the bar is locked to all incoming would-be patrons such tha bar employees and a select group of friends can sit around and drink without all the annoying cash-paying customers bugging them.
this guy kinda blew her off, so she was all like "i need to croosh someone NOW," drags me down the sidewalk and five minutes later we're in a lock in with 5 guys
by Landau December 04, 2006
 
3.
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.
by TestyWhenHungry April 23, 2011
 
4.
An overnight or extended period of time when a youth group is confined to a particular space in order for them to appreciate what might not have been expected.
The public library invited all middle schoolers to a lockin featuring a popular rock band.

Our youth group held a lockin last weekend with many cool events.
by Michael G. Terpstra, Ph.D. August 06, 2006
 
5.
popular slang for a homodyne with an extremely low pass filter, which making it very narrow band. Usually includes a frequency mixer, and not to be confused with bandpass filter.
"That lock-in is essentially shot-noise limited."

"I'm using the reference from that lock-in because I don't have a function generator."
by novakyu November 25, 2007