From The Propaganda Professor:
I first heard the term sky hook at age 19 when I worked briefly in a hardware store. As a hazing stunt against new employees, it was the custom for another employee to call the department where the newbie was working and, pretending to be a customer, ask about the availability of sky hooks (“in a brass or bronze finish”, my co-worker Joe specified when he called me). Never having heard the expression before, I assumed it was just a colorful term for some kind of hardware I was unfamiliar with; I did not assume that it was an item that bolted directly into the sky — which was indeed the idea. (After placing Joe on hold and figuring out I was being had, I picked the phone back up and said, “Sorry, sir, we have no brass or bronze — only raspberry.”)
The term sky hook originated among pilots in the early Twentieth Century as a tongue-in-cheek reference to some imaginary hook in the sky that held planes up and enabled them to maneuver under difficult circumstances. Metaphorically, then, it came to mean any theoretical concept that “held up” other concepts. (Subsequently, the label has been applied to other ideas, and even to real mechanical devices.) Or more broadly, any unsupported presumption that becomes the basis of policy or dogma. For example, the belief in a flat earth could be deemed a sky hook; from it was suspended the perception of how the entire cosmos looked and operated, and false notions about the limits of navigation. This is an example of top-down thinking; and it’s a good illustration of why it’s so counterproductive. (Still, top-down thinking has its uses in some circumstances; many people, for instance, who become very successful in their chosen field begin with a dream of where they want to end up and then figure out how to get there.)
The ultimate sky hook, of course, is God.