Something that is extremely annoying, frustrating, or insulting in a way that really bothers you.
From Middle English 'gallen' meaning 'to injure or make sore', derived from Old English 'gealla' (gall, a sore); originally referred to physical chafing or rubbing, then metaphorically extended to emotional irritation.
The word 'gall' literally refers to the bile produced by animals, which was thought to embody bitterness and anger—so calling something 'galling' is like saying it's as bitter and irritating as actual gall.
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