A castrated male sheep with grey wool, or a sarcastic reference to an old person or something worn out and useless.
From 'grey' plus 'wether' (a castrated male sheep), literally describing the coloring of this type of livestock.
The term 'greywether' carries an interesting dismissive attitude—calling something a greywether means it's past its prime and no longer useful, which says something about how agricultural societies viewed aging animals and perhaps people too.
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