I've been able to make the most sense out of "overthinking" by using an energy metaphor. A lot of NT people I've known seem to have to fight a sort of energy gradient to put them into a mental state where they can analyze or focus. In my case, my brain is permanently switched 'on' to that same mental state. For me, the energy gradient is in the other direction: it takes effort to turn my brain off. So relative to people such as these, I'm an "overthinker". I don't feel there's anything wrong with my brain, but by comparison, other people may feel I'm stressing unnecessarily or obsessing too much about something because their perception of me is that I'm exerting a great deal of energy getting my mental state up to a point where it can consider scenarios, anticipate conversations, etc.
On the other hand, maybe the point of the word is to suggest I'm thinking about something I perhaps shouldn't be, like social situations, in which case they may have a point. Still, I've accepted this part of me long ago when I decided that I'd rather not be around people who felt compelled to point out how much more than them I was thinking.
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"If people do not believe that mathematics is simple, it is only because they do not realize how complicated life is."
~~ John von Neumann