Mine is the volume of their voice. If all that they can do is just talk too loudly with no reason for it and don’t control the volume of their voice. That to me is a sign that you’re a stupid fuck. Because people think the louder they are, the more righteous and attention seeking they are. Than it is to just listen, digest and respond accordingly to anyone talking around you.
I will counter that the definition of intelligence is needed. Yours OP seems to be emo intelligence, maybe social intelligence. One can be very intelligent in other areas while not within these.
My quality would be a person’s ability to abstract and understand subjects through inference and intuition; to see all the layers of bigger pictures and the uncertainty of truth when contextualized.
In many ways, I fail at my own stupid, but it is a striving in life. After all, what use are achievable goals.
Stupid is subjective, circumstantial, and non binary.