man in the moon, n.¹ — Green’s Dictionary of Slang

man in the moon n.¹

also man of the moon

1. a watchman, a constable.

1630

J. Taylor ‘A Brood of Cormorants’ in Works (1869) III 9: Hee’s Lord High regent of the tedious night, / Man of the Moone he may be called right.

2. a nickname for the person, necessarily anonymous and quick to disappear, who pays out bribes at elections.

1866		Totness Election Petition ‘Evidence of Mr. Rob Harris.’ n.p.: I have had to deal with unknown gentlemen at Totnes before. A man in the moon is the natural consequence of a Totnes election [F&H].
1873		Sl. Dict. 222: Man in the moon the gentleman who is supposed to find the ‘pieces’ to pay election expenditure and electors’ expenses, so long as the latter vote his way.
1881		Contemp. Rev. xxxix 869: My labourers were paid in a public-house in the town by a man from behind a screen, who was invisible; after the fashion of the man in the moon, who pays bribes at elections [F&H].