1590s, from horse (n.) + chestnut. A tree probably native to Asia, introduced in England c. 1550; the name also was extended to similar North American species such as the buckeye. Said to have been so called because it was food for horses, "but this is appar. a mere guess" [Century Dictionary] and the sense is perhaps "large," as in horseradish. The nut resembles that of the edible chestnut but is bitter to the taste.