The phrase moonlight flit denotes a hurried departure at night, especially from rented accommodation to avoid payment of rent owed—cf. fly-by-night.

In this phrase, flit is a noun use of to flit, originally used in Scotland and Northern England to mean to move house or leave one’s home, especially secretly so as to escape creditors or obligations.

The phrase was moonlight flitting in Scottish use; it is first recorded in A complete collection of Scotish [sic] proverbs explained and made intelligible to the English reader (London, 1721), by James Kelly:

He has left the Key in the Cat hole.
He has left the Key under the Door.
He has taken a Moon light flitting.
He has gone without taking his leave.
I wat not what he has done with his Tripes, but he has taken his Heels.
These five are only proverbial Phrases, to signify that a Man has run away for fear of his Creditors: The last I heard only in Ireland, I suppose it is not used in Scotland.

The earliest instance of moonlight flit that I have found is from the Chester Courant, and Anglo-Welsh Gazette (Chester, Cheshire) of Tuesday 1^st April 1823, which reported that Messrs. Williams and Dicas, co-partners, as bankers and solicitors, at Holywell, in the county of Flint, brought an action to recover compensation in damages for a libel alleged to have been published by Mr. Taylor, the proprietor, and Mr. Garnett, the printer and publisher, of The Manchester Guardian, in an article of Saturday 7^th September 1822 on the evil tendency of provincial notes, which contained the following:

We ourselves saw a few days ago some notes, payable at a place in Wales, which had been issued by two vagabonds, one of whom was formerly a pettifogger in this town, who, after two years’ imprisonment for conspiring with a bankrupt to defraud his creditors—after being shunned by all decent men—after making from his last residence here a “moonlight flit,” to avoid a distraint for poor-rates, which he had appealed against on the ground of poverty—went immediately, in conjunction with the other wretch, to commence “banking.”