Fly-by-night and Spur-of-the-moment

Fly-by-night adjective - (of businesses and businessmen) unscrupulous.

Spur-of-the-moment and fly-by-night are semantically related. In some cases you can use "Spur-of-the-moment" instead an adjective phrase "Fly-by-night".

Spur-of-the-moment adjective - Made or done without previous thought or preparation.

Fly-by-night and spur-of-the-moment are semantically related. Sometimes you can use "Fly-by-night" instead an adjective phrase "Spur-of-the-moment".