What does it mean to be stupid?

I suppose it is pretty easy to define “stupid” as uneducated.
But I know lots of people who dropped out of school, and independently developed their minds to a level that I find intimidating.
On the other hand, in the neighborhood where I lived before my divorce, there was a PhD in Botany who maintained that plants had emotions. WTF!
So what are the criteria that would allow a reasonable person to render against another person the evaluation of “stupid”.