No Ed, you're not the only one who didn't know what this means, I'm positive that as Latin is not taught as much as it was 35 years ago, quite a few of us would openly admit to have forgotten what they were taught anyway, and as it doesn't affect my daily life, I'm not completely bothered either.
The only phrase that readily springs to mind is - Non compos mentis, I've always remembered that one - "not of sound mind and understanding"
It has always particularly applied to our Politicians.......:hehe: