I’ve been having some fun searching the French National Library for old records, books, articles, and journals for any mention of Chastain. It turns out chastain was also used to describe a certain brown, chestnut-like color. Close to ninety percent of the results returned were for the color and not the name. Here is just one example. This is from page 687 of the Universal Commercial Dictionary published in 1726.
A rough translation of the first entry for “chastaigne” is, “The fruit of a large tree, called chestnut.” The second entry for “chastaigne” or “chastain” states, “The color of chestnuts. Sometimes used to describe the color of silks or wool, but more often the latter. Can also be used to describe hair.”
The Online Etymology Dictionary has the following to say about the origins of chestnut:
chestnut (n.) 1560s, from chesten nut (1510s), with superfluous nut (n.) + Middle English chasteine, from Old French chastain (12c., Modern French châtaigne), from Latin castanea “chestnut, chestnut tree,” from Greek kastaneia, which the Greeks thought meant either “nut from Castanea” in Pontus, or “nut from Castana” in Thessaly, but probably both places are named for the trees, not the other way around, and the word is borrowed from a language of Asia Minor (compare Armenian kask “chestnut,” kaskeni “chestnut tree”). In reference to the dark reddish-brown color, 1650s. Applied to the horse-chestnut by 1832.
So chastain translates literally to chestnut in Old French (with origins around the twelfth century). Perhaps the surname Chastain originated with someone who had chestnut-colored hair? The other theory, which I mentioned in my first post, was that it was first used by someone who lived near a chestnut tree. (I suppose we shouldn’t rule out the combination of both theories – the first Chastain both lived near a chestnut tree and had chestnut-colored hair!)
As noted by the Online Etymology Dictionary, chastain was derived from the greek, kastaneia, and this greek word was derived from some other ancient tongue as evidenced by the similarities with the Armenian language. So while the surname Chastain may be eight to ten centuries old, the origins of the word itself stretch back thousands of years.