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

Flat

Fell flat as a damp squib.
—Anonymous

Flat as a pricked bladder.
—Anonymous

Flat as your hand.
—Anonymous

Flat as the beaten coin.
—Romance of Antar

Flat as a flounder.
—Beaumont and Fletcher

Lay flat like an anvil’s face.
—Robert Browning

Flat as a gravestone.
—Elizabeth Barrett Browning

Flat and bare as Hebrew verse of Bishop Hare.
—John Byrom

Flat as a fillet of sole.
—Irvin S. Cobb

Flat as a juryman.
—Charles Dickens

Flat as a flail.
—Bret Harte

As flat as a pancake.
—Ludvig Holberg (1684–1754)

Flat as a rose that has long been pressed.
—Oliver Wendell Holmes

As flat as Æschylus in Bohn’s translation.
—Victor Hugo

Flat as a willow-pattern plate.
—Victor Hugo

Flat as an excuse.
—Sydney Munden

Flat as the fens of Holland.
—Sir Walter Scott

Flat
As dead sands be at utmost ebb that drink
The drainèd salt o’ the sea.
—Algernon Charles Swinburne