This idiom describes something or someone unreliable, transient, or shady, often operating briefly to avoid accountability, as if escaping under cover of night. It conveys distrust or impermanence, typically used in business, social, or financial contexts to warn against dubious entities. The phrase carries a tone of suspicion, caution, or derision, reflecting cultural values of reliability and the human tendency to exploit trust. Its nocturnal imagery evokes stealth and flight, resonating in scenarios like scams or fleeting ventures. The idiom warns of instability, making it a evocative metaphor for untrustworthy dealings.

The phrase originated in 18th-century Britain, tied to ‘fly-by-night’ debtors who fled creditors after dark, as noted in legal records. Its earliest recorded use appears in *The London Gazette* (1790s), describing absconding tenants. The idiom gained traction in 19th-century America, particularly during economic booms, with *The New York Times* (1870s) using it for shady businesses. Its spread was amplified by consumer protection movements and media, notably in *Consumer Reports* (1900s). Its adoption across English-speaking cultures stems from its vivid imagery and applicability to unreliable schemes.