blue-sky v. to imagine or propose unreasonable or as-yet unfeasible ideas. (source: Double-Tongued Dictionary)
Further reading
For at least three centuries, declaring I’ll eat my hat! has indicated that the speaker is so certain they’re right, that if they’re not, they’re willing to swallow their chapeau. Variations of this phrase include I’ll eat my boots, I’ll eat my cap...
The phrase throw in the towel meaning “give up,” stems from boxing competitions in the late 19th. If a boxer’s team wanted to stop the fight—usually because their fighter was taking too much punishment—they would literally throw a towel into the air...
There is a much earlier use of “blue sky” than is referred to here. Securities law has long used “blue sky” to refer to the legal regulation of worthless securities. The first state “blue sky” law was enacted in Kansas in 1911. One scholar notes that the state “blue sky” laws were to counter the issuance of fraudulent securities which had no more value than the blue sky.
Yes, that’s exactly right. On this site, I try to cover terms which are under- or undocumented, so, since the sense is different, I didn’t cover it here. The Oxford English Dictionary, by the way, has a cite of the securities-related blue sky to 1906.
I would argue that your use of “blue sky” is well within the sense of the older term. Usage within securities law focused on the “worthless” nature of the securities. Your emphasis on “unreasonable” or “unfeasible” seem to be an entirely reasonable or feasible extension of the long-accepted use. Please do keep up the good work. I was pleased to learn of your web site on NPR’s Talk of the Nation program today.
I have traced the use of the term “blue-sky” in connection with securites to the 1890s, and have cited verifiable references in my glossary called the “GelberLaw Glossary” on my website at http://www.GelberLaw.net