- A creature which flies at night; a nocturnal flier or traveler.
- One who departs or flees at night in order to avoid creditors, law enforcement etc. (often used attributively).
- Do not give your credit card number to that fly-by-night operation.
- (idiomatic, derogatory) A person or business that appears and disappears rapidly, or gives an impression of transience.
- (idiomatic, derogatory) A traveling businessman or tradesman.
Usage notes 
The phrase implies that the quality of work done or goods sold is shoddy enough that the person responsible must leave town under cover of darkness to evade angry customers, or sometimes legal authorities. The earliest use of the phrase is said to have been a reproach for women, signifying that she was a witch. The modern usage first appeared in 1823.
- "Online Etymology Dictionary: fly-by-night", Douglas Harper. URL accessed on 2011-05-21.