GALL (a word common to many Teutonic languages, cf. Dutch gal, and Ger. Galle; the Indo-European root appears in Gr. and Lat. fel; possibly connected with "yellow," with reference to the colour of bile), the secretion of the liver known as "bile," the term being also used of the pear-shaped diverticulum of the bile-duct, which forms a reservoir for the bile, more generally known as the "gall-bladder" (see Liver). From the extreme bitterness of the secretion, "gall," like the Lat. fel, is used for anything extremely bitter, whether actually or metaphorically. From the idea that the gall-bladder was the dominating organ of a bitter, sharp temperament, "gall" was formerly used in English for such a spirit, and also for one very ready to resent injuries. It thus survives in American slang, with the meaning "impudence" or "assurance."
"Gall," meaning a sore or painful swelling, especially on a horse, may be the same word, derived from an early use of the word as meaning "poison." On the other hand, in Romanic languages, the Fr. galle, Sp. agalla, a wind-gall or puffy distension of the synovial bursa on the fetlock joint of a horse, is derived from the Lat. galla, oak-apple, from which comes the English "gall," meaning an excrescence on trees caused by certain insects. (See Galls.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)