Where does “blæthræ” come from?

blæthræ (Old Danish) comes from Old Norse blaðra, from Proto-Germanic blēdrǭ, from Proto-Indo-European bʰleh₁dʰreh₂(n).

Ancestry of “blæthræ”, step by step

Step	Language	Word	Meaning
1	Old Norse	blaðra	bladder
2	Proto-Germanic	blēdrǭ	bladder
3	Proto-Indo-European	bʰleh₁dʰreh₂(n)	—

blæthræ — Etymology, Origin & Word History | Etymology Explorer