Where does “phonno” come from?

phonno (Old High German) comes from Vulgar Latin faonius, from Latin favōnius, from Latin faveō, from Proto-Indo-European gʰoweh₁ — to notice.

Ancestry of “phonno”, step by step

Step	Language	Word	Meaning
1	Vulgar Latin	faonius	—
2	Latin	favōnius	—
3	Latin	faveō	to be favorable, to be well disposed or inclined towards, to favor, promote, befriend, countenance, protect
4	Proto-Indo-European	gʰoweh₁	to notice