E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
So called because they were once supposed to increase the butter of milk. No doubt those cows give the best milk that pasture in fields where buttercups abound, not because these flowers produce butter, but because they grow only on sound, dry, old pastures, which afford the best food. Miller, in his Gardeners Dictionary, says they were so called under the notion that the yellow colour of butter is owing to these plants.