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1.
The true usage of the phrase is to claim that you simply 'do not know', however, the phrase is often interpreted as 'Fuck Nose' which may cause offence to the listner. This mis-interpretation is often replied to using 'Cunt Face' which is regarded as the appropriate answer.
"What time is Adam supposed be be here?
"Fuck Knows"
"Only asking...Cunt Face"
von Mystick Havok 21. Januar 2008
 
2.
An abbreviation or slang used to by many for a way to say "I don't know"
Boy1: Dude, what's the time?
Boy2: Fuck knows
von Dalmas 2. Januar 2014