Forrest Gump's "Stupid is as stupid does" is a variant of an old
adage, "Handsome is as handsome does." This saying appears in J.R.R.
Tolkien's "The Lord of the Rings" and in Herman Melville's "Billy
Budd," and can be traced as far back as the 14th Century.

"Handsome is as handsome does" basically means that true handsomeness
has to do with a person's behavior, not just a handsome face. The
saying is also phrased in the forms "Pretty is as pretty does" and
"Beauty is as beauty does."

Forrest's version of the saying means that stupidity is not just a
surface thing derived from a person's appearance. Stupidity is a
matter of deeds, not looks. Like the other versions, it comes down to
this: judge people by what they do, not by how they appear.

"HANDSOME (PRETTY) IS AS HANDSOME (PRETTY) DOES - Good deeds are more
important than good looks. The proverb was first recorded by Chaucer
in 'The Wife of Bath's Tale' (c. 1387). In 1766, in the preface to
'The Vicar of Wakefield,' Oliver Goldsmith wrote: 'Handsome is that
handsome does.' First attested in the United States in 'Journal of a
Lady of Quality' (1774). The saying is found in varying forms,
including 'Beauty is as beauty does'." From the 'Random House
Dictionary of Popular Proverbs and Sayings' by Gregory Y. Titelman
(Random House, New York, 1996)."