Where does the term “Blue Sky Law” come from and What does Blue-Sky Law mean?

A blue-sky law is any law enacted for the protection of suckers who, without some such safeguard, might part with cash for the purchase of hot air, scenery, or even the blue sky from unscrupulous promoters.

The first of the laws that acquired the term was enacted in Kansas in 1912.

Our “blue sky” in such connection is the equivalent of the German blauer Dunst, literally “blue haze,” but in colloquial speech, “humbug.”