Where does “forsaketh” come from?

forsaketh (English) comes from English -eth, from Middle English -eth, from Old English -eþ, from Proto-Germanic -þi, from Proto-Indo-European -ti.

Ancestry of “forsaketh”, step by step

Step	Language	Word	Meaning
1	English	-eth	"goeth, "playeth; used to create ordinal numbers...
2	Middle English	-eth	Used to form the third-person singular present...
3	Old English	-eþ	Third person singular present indicative ending of class 1 weak verbs
4	Proto-Germanic	-þi	—
5	Proto-Indo-European	-ti	—