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"
Essentially means noone knows what's going to happen.
Derived from a quote from the Netflix series Ozark
It has become widely used in the cryptocurrency sphere as a phrase to remind people you can't know if the market will go up or down.
Redditor 1: Did you guys buy the dip? I bought LRC and CRO
Redditor 2: This is not a dip, this is the beginning of bear market
Redditor 3: Nobody knows shit about fuck.