Fitted with a girth (the band around a horse's belly) or having been measured around the circumference.
From 'girth' plus '-ed' suffix, where 'girth' comes from Old Norse 'gjörð' meaning a belt or band. Shows how Old Norse vocabulary penetrated English through Viking contact.
Choosing the right girth size for a horse was genuinely important—too loose and the saddle slips, too tight and the horse can't breathe. This word would have been part of every stable worker's daily vocabulary, yet most modern people have never heard it.
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