What's the meaning of the phrase 'Fly-by-night'?
A 'fly-by-night' is someone who attempts to evade responsibilities or debts by absconding under cover of darkness. Typically, this may be covertly moving out pf a property to avoid paying the rent.
More generally, 'fly-by-night' is applied to any person or business of a poor 'here today; gone tomorrow' reputation.
What's the origin of the phrase 'Fly-by-night'?
When 'fly-by-night' was first coined in the 18th century it was with a different meaning from the above. A 'fly-by'night' was a colloquial term for a witch.
There is no better chronicler of English slang terms of the Georgian period than Captain Francis Grose and he is indeed the first to record this expression in the 1788 edition of his Classical Dictionary of the Vulgar Tongue:
FLY-BY-NIGHT. You old fly-by-night; an ancient term of reproach to an old woman, signifying that she was a witch, and alluding to the nocturnal excursions attributed to witches, who were supposed to fly abroad to their meetings, mounted on brooms.
How the term changed in meaning from the specific and literal 'witch' to its present usage isn't clear. The original form does suggest that the name was used as a slur. It's quite possible that, on hearing the term, a someone might later use it with a similar negative sense to refer to anyone else who had disappeared overnight.