fly-by-night

American  

[flahy-bahy-nahyt] / ˈflaɪ baɪˌnaɪt /

adjective

1. not reliable or responsible, especially in business; untrustworthy.

   a fly-by-night operation.

2. not lasting; brief; impermanent; transitory.

   a fly-by-night theater.

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noun

1. a person or thing that is unreliable, especially a debtor who evades or attempts to evade creditors.

2. a person regarded as a poor credit risk.

fly-by-night British  

adjective

1. unreliable or untrustworthy, esp in finance

2. brief; impermanent

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noun

1. an untrustworthy person, esp one who departs secretly or by night to avoid paying debts

2. a person who goes out at night to places of entertainment

fly-by-night Cultural  

1. Shady or untrustworthy: “Before buying stock in a newly formed company, the prudent investor will check its owners' credentials to make sure it's not a fly-by-night operation.”

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Etymology

Origin of fly-by-night

First recorded in 1790–1800

Example Sentences

That they had been friends for so long before, Ms. Röhm said, made it obvious that their romantic relationship was not a fly by night affair.

From New York Times • Nov. 12, 2021

Nowadays they sleep by day and fly by night.

From New York Times • Dec. 11, 2020

H&R Block probably weeds out most of these but I worry about some of the other "fly by night" tax preparation places. 

From Time • Feb. 9, 2013

Rushing the season, all sorts of wildly sanguine early birds fly by night only to crash by morning.

From Time Magazine Archive

Moths are Scale-wings that fly by night, and have switch or feather-shaped feelers; they keep their wings spread open when they alight, and in the bundle-baby stage, they are wrapped in a cocoon.

From Woodland Tales by Seton, Ernest Thompson