man in the moon

Meanings

1. An image of a man perceived in the dark maria (plains or "seas") and light highlands or other features of the Moon, originally regarded as a man with a burden on his back or accompanied by a small dog, and now more commonly as a man's face in the full moon or his profile in a crescent moon; hence, an imaginary man thought to be living on the Moon.
2. An imaginary person; also (UK politics, slang), an unidentified person who illegally pays for election expenditure and electors' expenses, as long as the latter vote as the person wishes.

Etymology

From man + in + the + moon.