E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Muff (A).
A dull, stupid person. Sir Henry Muff, one of the candidates in Dudley’s interlude, called The Rival Candidates (1774), is a stupid, blundering dolt. He is not only unsuccessful in his election, but he finds that his daughter has engaged herself during his absence.
