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The time at which caffeine is required to commence or continue mental and/or social function. Most common towards the end of the week or during a night shift.

See also "half past caffeine"
Person 1: "You've been staring at that screen for an hour, and haven't done a thing. You know this work's due in an hour right?"

Person 2: "Yeah, I got like four hours sleep and can't concentrate. It's definitely caffeine o'clock."
by archangel62 May 28, 2009
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