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

Silly

Silly as calves.
—Anonymous

Silly as an old maid at a marriage.
—William Congreve

You look … as silly as a tumbler when he’s been upside down and has got on his heels again.
—George Eliot

Silly as an owlish roysterer’s glazed stare at the young aurora.
—George Meredith

Silly as a booby.
—Mason L. Weems