What is "fly-by-nights"

Usage examples of "fly-by-nights".

Now she works in a drop-in business shop, stacking shelves for virtual fly-by-nights that come and go like tourists in the Festival season—but humans aren’t in demand for shelf stacking either, these days.

All you got now is losers or fly-by-nights gittin’ in their time for the surgical boards and fixin’ to leave the first time they got a grand in the bank.