“ 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.” ”