A wild plant (also called cow-herb) with small pink or white flowers and opposite leaves, found in pastures and considered a weed or rangeland plant.
Compound word from 'cow' (indicating association with pastures and cattle) plus 'herb'. The plant is called cowherb because it grows commonly in cow pastures throughout temperate regions.
Cowherb is a plant named purely for where it grows—not because cows eat it or it affects them, but simply because it's the herb you find where cows graze, showing how pastoral vocabulary was purely locational.
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