Term coined by ledgendary ambassadors 'Susan' and 'David' Siboo, parents of the late Ashweh Siboo. However misleading it may be, the phrase was rareley used at 11 o'clock GMT, or at times reasonably close to this hour. Some beleive that the prime users followed multiple 'time zones' at one time.
Today, it usage has broadend. 1.it is used primarily to express frustration for the the unwelcomed arrival of someone and 2. A shortend 'command' if you will, ordering the subject to go to the cinema with the subject at that time. It's usage is favuored especially when the subject has an important event, a 'test' for example, the following day, in order to deprive them from sleep and revision. The commanded version of '11 o'clock' is surprisingly used at 11 o'clock GMT.
1. *doorbell rings at 9:52pm GMT*
Joe Thompson: Hi, is Ashweh there?
David Siboo: WHAT THE FUCKING FUCK?!! ITS 11 O'CLOCK!!!!
Joe Thompson: But it's not even 10 o'clo..
David Siboo: SHUT THE FUCK UP YOU FUCKING FAG.
*door slams*

2. Susan Siboo: Ashweh, 11 o'clock.
Ashweh: Ok, dont worry, i dont need much sleep today, ive only got a test tommorow
by Molten 'fucking' Slag June 8, 2004
The time that a party really "begins", no matter what scheduled time the party was actually supposed to start previous. This is the time that, without fail, droves of people show up all at once and a party can go from almost non-existent to full-fledged rager.
"Man, this party was so lame for like the past 3 hours, now it's insane! (Looking down at watch, seeing it's around 11 PM) 11 O'Clock Rule, happens every time!"
by Matty Rob April 6, 2009
Girls that say they are down to party and show up, when actually they aren't at all and have to leave early.
Guy#1: Man that girl over there is fine is she DTF?
Guy#2: Don't even think about it, i heard she was an 11 o'clock girl.

Rees: There sure were a lot of 11 o'clock girls at kevins.
Logan: Tell me about it.
by Parrish17 April 5, 2010
If you're up at 11 on a school night, this rule states that your choices become to either whack your dick or go to sleep.
Man, I had myself an 11 o'clock smack last night because I wasn't ready to go to bed.

(talking to prissy girl) Well, it's the 11 o'clock smack, got to go to bed.
by ziggytheman October 24, 2014
You are not allowed to get drunk before eleven
You have to take me home

But what am i going to do if i am not drunk before eleven

So you have to get drunk before 11

Yeah its the 11 o'clock rule
by burnt cracker March 17, 2010