Where does “fourtethe” come from?

fourtethe (Middle English) comes from Old English fēowertēoþa, from Proto-Germanic *feurþō tehundō, from Proto-Germanic *fedurþô tehundô.

Ancestry of “fourtethe”, step by step

Step	Language	Word	Meaning
1	Old English	fēowertēoþa	—
2	Proto-Germanic	*feurþō tehundō	—
3	Proto-Germanic	*fedurþô tehundô	—