Where does “*fak” come from?

*fak (Proto-Germanic) comes from Proto-Germanic faką, from Proto-Indo-European peh₂ǵ- — to attach.

Ancestry of “*fak”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	faką	An enclosed or defined space; division;...
2	Proto-Indo-European	peh₂ǵ-	to attach