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

Dull

Dull as a beetle.
—Anonymous

Dull as a convent.
—Anonymous

Dull as a Dutchman.
—Anonymous

As dull as a hoe.
—Anonymous

Dull as a post.
—Anonymous

Dull as a Quaker meeting.
—Anonymous

Dull as cloudy skies.
—Anonymous

Dull as mutes at a funeral.
—Anonymous

Dull as ditch water.
—Anonymous

As dull as the debates of Dutch burgomasters on cheese parings and candle ends.
—Anonymous

Dull as Lethe.
—Anonymous

Dull as a dormouse.
—Beaumont and Fletcher

Dull as the earth.
—Beaumont and Fletcher

Dull as sin.
—Samuel Laman Blanchard

Dull as lead.
—Anne Brontë

Dull as any London afternoon.
—Elizabeth Barrett Browning

Dull as an archdeacon.
—Gilbert K. Chesterton

Dull as laudanum.
—Charles Dickens

Dull as an ox.
—Henry Fielding

With eyes as dull as smoky glass.
—Norman Gale

Dull as a post.
—John Gay

Dull as a bachelor beaver.
—Sam Slick

Dull as a boiled codfish.
—Sam Slick

Dull as a whetstone.
—Robert Heath

Dull as a pig of lead.
—Help to Discourse

Dull as a mud-flat.
—Maurice Hewlett

Dull as an alderman at church, or a fat lap-dog after dinner.
—Thomas Holcroft

Dull as a donkey.
—Thomas Hood

Dull as lead.
—Andrew Lang

Dull as a tract.
—George Meredith

Dull as night.
—William Shakespeare

Duller than a great thaw.
—William Shakespeare

Dull as catalogues.
—Richard Brinsley Sheridan

Dull as a sheep.
—Robert Louis Stevenson

Sound as dull as unstrung drum.
—James Sully

Dull as the dead fume of a fallen fire.
—Algernon Charles Swinburne

Dark and dull like the mould upon a skull.
—Frank Waters

Dull as a platonic lover.
—Woman Turned Bully

Dull as a country squire.
—William Wycherley