What is the meaning of the saying 'stupid is stupid does'?

"Stupid is as stupid does" indicates that a person is judged stupid by the stupid acts they commit.

A1. The phrase "beauty is as beauty does" is a common southern expression. It conveys the notion of judging someone on their actions as opposed to superficial appearance. The mom (played by Sally Fields) in the movie Forrest Gump twisted this saying into "stupid is as stupid does" as way of placating her slow-witted son. It is sort of a meaningless expression, and generated a laugh at theaters in the south. The play on phrases was apparently lost in other regions where the original saying was not known.

A2. Stupidity is demonstrated by one's actions.

A3. It means that although you may be slow, stupidity is based more on your actions than on the speed at which you think.

A4. It means that people think of you as stupid if you act stupid or perform stupid acts.