From the Latin name “Castanea” which means
“chestnut” and refers to dark reddish-brown colour. (the
Latin "Castanea" derives f rom the Greek. "kastaneia",
which meant either "nut from Castanea" in Pontus (eastern Greece)
or "nut from Castana" in Thessaly, but probably both places are
named for the trees, and the word is probably borrowed from a language
of Asia Minor.
( The specific name implies:
"coloured like a chestnut").