An American plant, or rather genus of plants, named after Quassi, a negro.
“Linnaeus applied this name to a tree of Surinam in honour of a negro, Quassi, who employed its bark as a remedy for fever; and enjoyed such a reputation among the natives as to be almost worshipped by some,” —Lindley and Moore: Treatise of Botany, part ii. p. 947.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894