Where does “af” come from?

af (Old Frisian) comes from Proto-Germanic ab, from Proto-Indo-European h₂epo — from, off, back; off, away; off, from.

Ancestry of “af”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	ab	away from; away
2	Proto-Indo-European	h₂epo	from, off, back; off, away; off, from