"bile," O.E. galla (Anglian), gealla (W. Saxon), from P.Gmc. *gallon- (cf. O.N. gall, O.H.G. galla), from PIE base *ghol-/*ghel- "gold, yellow, yellowish-green" (cf. Gk. khole, see cholera; L. fel; perhaps also O.E. geolo "yellow," Gk. khloros). Informal sense of "impudence, boldness" first recorded Amer.Eng. 1882; but meaning "embittered spirit, rancor" is from c.1200.
"sore spot on a horse," O.E. gealla "painful swelling," from L. galla "gall, lump on plant," originally "oak apple," of uncertain origin. Perhaps from or influenced by gall (1) on notion of "poison-sore." The verb meaning "to make sore by chafing" is from c.1440; fig. sense of "harass, irritate" is from 1573.