not reliable or responsible, especially in business; untrustworthy: a fly-by-night operation.
not lasting; brief; impermanent; transitory: a fly-by-night theater.
noun Also, fly-by-nighter.
a person or thing that is unreliable, especially a debtor who evades or attempts to evade creditors.
a person regarded as a poor credit risk.

1790–1800 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
Cite This Source Link To fly-by-night
World English Dictionary
1.  unreliable or untrustworthy, esp in finance
2.  brief; impermanent
3.  an untrustworthy person, esp one who departs secretly or by night to avoid paying debts
4.  a person who goes out at night to places of entertainment

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1796, slang, said to be an old term of reproach to a woman signifying that she was a witch; extended 1823 to "anyone who departs hastily from a recent activity," especially while owing money. The two senses involve the two verbs fly.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Cultural Dictionary

fly-by-night definition

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.”

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
To restore confidence, hedge funds have tried to shed their reputation for
  being fly-by-night operations.
The result is a fly-by-night operation based on guesses, rather than
  knowledgeable decisions.
Not to mention fly-by-night operations dumping unsafe products for a quick
  profit and heading for the hills.
We're not talking about some fly-by-night operation either.
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