Sentence:  The automobile industry rose almost overnight. Orders poured into the offices of companies already organized; new companies were formed by dozens, capitalized at millions of dollars. <WSD>Fly-by-night</WSD> concerns sprang up like mushrooms, flooded the country with stock-selling schemes, established factories where parts of motor cars, bought elsewhere, were assembled. 
Meaning of the word marked with <WSD> tags:  A person or business that appears and/or disappears rapidly, appearing untrustworthy, or giving an impression of transience.