English
mare's fart
From mare + s + fart.
Proto-Indo-European *márkos ‘horse’
…
Proto-Germanic *marhijō ‘female horse’
Proto-West Germanic *marhijā ‘mare, female horse’
Old English mīere ‘female horse, mare’
Middle English mare ‘a horse’
English mare
Proto-Indo-European *perd- ‘to fart loudly, pass gas, break wind’
Proto-Germanic *fertaną ‘to break wind, fart’
Proto-West Germanic *fertan
Old English *feortan ‘to fart’
Middle English ferten
English fart
The story
A nickname stemming from the unpleasant smell of the plant's leaves.
Today it means
noun Ragwort (Jacobaea vulgaris).
Language periods are conventional ranges, not dates for this word.
Etymograph / mare's fart