Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Stupid

Stupid as an excuse.
—Anonymous

Stupid as a sloth.
—Anonymous

Stupid as a fact.
—Honoré de Balzac

Stupid as a stone.
—Robert Browning

Loftily stupid, like dumb idols.
—Thomas Carlyle

Stupid as a coot.
—Frank Cowan (Dictionary of Sea Proverbs)

Stupid as a little downy owl.
—George Du Maurier

Stupid … like Falstaff in his old age.
—Thomas H. Huxley

Stupid as a log.
—Isaac Jackman

Stupid as hounds chasing an iron deer.
—Sydney Munden

Stupid as a post.
—Clement Robinson

Stupid as the swine.
—Hans Sachs

Stupid as a hog.
—Leo Tolstoy