Stupid is as stupid does

[Tina]

It means that an intelligent person who does stupid things is still stupid. You are what you do.
Another variation is beauty is as beauty does or ugly is as ugly does.

[PussyCat]

a catch-all term that basically means that stupid people are gonna keep on doing stupid things because thats what they are, period. The term was popularized in the [1994] [Tom Hanks] 3-hour blockbuster movie [Forrest Gump].