Where does “bleth” come from?

bleth (Middle English) comes from Old English blēaþ, from Proto-Germanic blauþuz, from Proto-Indo-European mlew-.

Ancestry of “bleth”, step by step

Step	Language	Word	Meaning
1	Old English	blēaþ	gentle, timid, peaceful, inactive; gentle, shy,...
2	Proto-Germanic	blauþuz	weak, feeble, emaciated; lacking, void, naked;...
3	Proto-Indo-European	mlew-	—

bleth — Etymology, Origin & Word History | Etymology Explorer