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

Serious

Serious as a philosopher.
—Miles P. Andrews

Serious as an owl.
—Anonymous

Serious as a portmanteau.
—Anonymous

Serious as a doctor.
—Alexandre Dumas, père

Serious … as one would whisper that a lion’s near.
—Thomas Hood

Serious as the fifth act of a tragedy.
—Joseph Jefferson

Serious as taxes.
—George Henry Lewes

Serious as a pope.
—Francisque Sarcey