A bag of food or grain placed over a horse's mouth so the animal can eat while working, or colloquially, food in general.
From 'feed' plus 'bag,' a compound that emerged in the 19th century for the practical device used on farms and with working animals, later becoming slang for 'eating heavily.'
The phrase 'put on the feedbag' became American slang for eating heartily, probably because horses wearing feedbags seemed to be dedicated to serious consumption—it's the kind of folk etymology that shows how practical farm terms became colorful figures of speech.
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