Stupid is as stupid does is one of the ultimate comebacks of history, especially common on kindergarten yards and playgrounds, swarmed by arguing children.
The phrase basically means, that stupid people are identified by their actions, rather than their mere appearances.
The phrase had been part of the English language for quite a while, at least since the 19th century, though it is hard to tell with these children’s idioms.
It is one of the first comebacks, mothers expand the armory of their children with, in order for them to protect themselves from meaner kids’ insults.
The term got massively popularized in 1994, when it became one of the iconic catchphrases of the legendary motion picture, Forrest Gump.
In it Forrest, despite being an obvious simpleton, protects his dignity with the phrase several times, providing a mighty example for us all; despite being slightly simple in the mind, we may still be regarded as a sane person, through our actions.
Urban Dictionary had added the expression to their definitions as early as 2006, helping it spread on the intertwined webs of the internet.