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1.
The time at which it is most appropriate that one gets the fuck out/off
Hey! Guess what time it is: it's GTF O'Clock! Get the fuck out of my house!
by Skilhelm January 16, 2011
 
2.
The time the clock in the office displays when it's clear that your employer is going under.
They canceled my approved vacation day 'cause we have to move all the servers out of the office space we're renting by the end of the month. Sounds like it's GTFO'clock.
by blumer November 25, 2008