Here's the worst: Learning curve.
"This program has a steep learning curve!"
Yes, that means it's easy to learn.
"No, I mean that it's hard to learn."
Plot knowledge vs time. If it takes a long time to acquire a particular level of knowledge, the slope of that line will be shallow. If knowledge is acquired in a short time, the slope will go upward sharply.
I disagree. The saying is quite OK, but you are misunderstanding it.
There is a logic behind this saying:
Steep learning curve means, you need to acquire new knowledge at a massive rate, all the time, and as a result, cannot get any work done. All your effort is going into the learning process.
Flat learning curve means, you slowly acquire some new knowledge as you go, getting better with the tool, while still getting work done.
Finally, whether it's finally easy or difficult to fully learn, the slope of the curve is irrelevant. The
height of the
required target level is; the amount of knowledge that needs to be acquired. The saying (and your version of just inverting it) completely ignores this aspect.
"Steep learning curve" saying doesn't focus on the total complexity, but it describes the initial amount of involvement that's needed to be able to do
anything with the tool: you are learning
the tool quickly, but only because you
have to. The opposite is the flat learning curve: you are not learning
the tool, you are just using it (and doing the work).
That's why it's called "learning curve", not "getting work done curve". Learning isn't always a positive; you have limited resources for learning and people hate to spend time learning tools when they want to use the tools for the job.
Given that people have some limited rate of putting effort into a project per time, this defines a maximum slope. If you plot two curves, "learning curve for the tool" and "advancing the actual project", the sum of their slopes is somewhat constant. If the learning curve is steep, the "advancing the project" curve has to be flat, and vice versa.
The massively important distinction is, whenever the "learning" part is really important, then it has to be done, and having a steep curve is acceptable. Still, having it
too steep may cause psychological problems of totally losing focus of the problem. Whenever the "learning" part is unintended - for example, you definitely do not plan to be a PCB designer, yet have to use a PCB EDA tool for some weird reason for a small project - this steep learning curve is pretty much unwanted.