Where does “*kastannjā” come from?

*kastannjā (Proto-Germanic) comes from Latin castanea, from Ancient Greek καστάνεια, from Ancient Greek κᾰ́στᾰνᾰ — sweet chestnuts; chestnut.

Ancestry of “*kastannjā”, step by step

Step	Language	Word	Meaning
1	Latin	castanea	a chestnut tree; a chestnut
2	Ancient Greek	καστάνεια	Alternative form of κᾰ́στᾰνᾰ; chestnut
3	Ancient Greek	κᾰ́στᾰνᾰ	sweet chestnuts; chestnut