The Brief: Fak is slang for the F-word.

DEEP DIVE
Variations include fakka and faka. It is often used as a replacement for the F-word in places where cursing is prohibited.
Fak, I'm not ready for college.
— Andi (@andidomengsu_) August 4, 2018
"I'm gonna go to bed early"
*nearly midnight*
"Fak" π— Walker Layne (@TexasRanger6290) August 2, 2018