The hourglass model and narrow waist design are the same thing. The "narrow waist" refers to the neck of an hourglass.
According to this video the original source is 👤 Jon Postel splitting up the design of TCP and IP.
It quotes Postel in IEN # 2 from 1977:
We are screwing up in our design of internet protocols by violating the principle of layering. Specifically we are trying to use TCP to do two things: serve as a host level end to end protocol, and to serve as an internet packaging and routing protocol. These two things should be provided in a layered and modular way. I suggest that a new distinct internetwork protocol is needed, and that TCP be used strictly as a host level end to end protocol.
At 3:47 in the video the speaker describes Postel drawing an hourglass and using the term "narrow waist" (emphasis mine):
The starting point for the Internet was John saying, "The structure of our world is this: there's only one piece-- that thing in the middle-- that has to be everywhere. Everything else is sitting on individual hosts or individual wires. The thing that has to be everywhere you want it to be absolutely bulletproof, you want it to be really simple, and you want it to be a foundation that you can build on above and below. And the foundation's not going to ever change, because if it changes everything that rests on it is going to have to change and that's going to really stifle the innovation". So John drew an hourglass and said, "Look this is the way the world should be shaped. We get a narrow waist in the center which has to be everywhere. Above it we can put our transport protocols more the better lots of experimentation. Below it we can put lots of different interfaces the more the better lots of experimentation. In the middle we're going to do this thing that's going to be as simple as we can possibly make it and we're never going to let it change."
So the original hourglass metaphor seems to be for systems that (1) have layered architectures, (2) follow a complex-simple-complex layering, and (3) hope not to have to change the simple layer much.
I heard about the Van Jacobson video above while reading the article The Internet Was Designed With a Narrow Waist.