E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
In many places statute fairs are held, where servants seek to be hired. Carters fasten to their hats a piece of whipcord; shepherds, a lock of wool; grooms, a piece of sponge, etc. When hired they mount a cockade with streamers. Some few days after the statute fair, a second, called a Mop, is held for the benefit of those not already hired. This fair mops or wipes up the refuse of the statute fair, carrying away the dregs of the servants left.