The Etymology of Fat
Fat is grammatically a fossil. In Old English fǣtt was the past participle of the verb fǣtan, which meant to fatten or cram (an animal for slaughter, or a person for feasting). The verb itself died out, but the past participle survived as a free-standing adjective — so fat literally means having been fattened, a frozen grammatical form whose verbal source has long disappeared. The Proto-Germanic ancestor *faitida- is shared with Old Norse feitr, German fett, Dutch vet, and Swedish fett, all meaning fat. Behind that lies a Proto-Indo-European root *poid- (to be plump), distantly related to Greek pīwon (fat) and Sanskrit pīna (fat). Compound and slang uses are ancient and continuous: the fat of the land (Genesis 45:18, in the King James of 1611), to live off the fat of the land, fathead (1842), fat cat (1928, originally American politics), and fat lot (sarcastic, 1892).