I wish there was a clinical definition of stupid, but there isn't really. Anyone can be stupid. Moreover, stupid is curable. The admonishment "Don't be stupid" suggests that a person can quickly change the way they think about something. Retarded, with its clinical definition, can't be avoided just by wishful thinking.

If there were any way at all of determining what a person's maximum ability would be during their lifetime, then I suppose failing to reach that pinnacle could be considered stupidity. But then, the movie Gattaca had pretty good commentary about the dangers of defining such limits. "What if somebody exceeds their (genetic) potential?"

Maybe stupid is willfully choosing ignorance on a matter. Maybe that's a useful working definition.