fly-by-night
fly-by-night (1) {adj.}
Set up to make a lot of money in a hurry, then disappear so people can't find you to complain about poor work, etc.; not trustworthy; not reliable.
Mrs. Blank bought her vacuum cleaner from a new company; when she tried to have it fixed, she found it was a fly-by-night business.
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Source: A Dictionary of American Idioms
