Frank J. Wilstach, comp. A Dictionary of Similes. 1916.
Secret (Adjective)
Secret as rocks under sea.
—Thomas Lovell Beddoes
Secret as the grave.
—Lord Byron
Secret as the head of Nilus.
—William Congreve
Secret as the night.
—William Congreve
Secret as thought.
—Francis Fawkes
Secret as a confessor.
—John Gay
More secret than a nest of nightingales.
—John Keats
Secreter than the isle of Delos.
—Anonymous
Secret as the magnet-stone.
—George P. Lathrop
Secret as your midwife.
—Edward Sharpham
Secret as a coach-horse.
—Richard Brinsley Sheridan