A corruption of boc. German, buche, beech-wheat; it is so called because it is triangular, like beech-mast. The botanical name is Fagopyrum (beech-wheat).
The buckwheat Whitened broad acres, sweetening with its flowers The August wind.
Bryant: The Fountain, stanza 7.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894