Solemnity is the fool’s shield and buckler.

[La gravité est la bouclier des sots.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Pensées Diverses [Assorted Thoughts], # 1637 / 1051 (1720-1755) [ed. Guterman (1963)]
    (Source)

(Source (French), # 1051). Other translations:

Gravity is the shield of fools.
[tr. Clark (2012), # 1637]

Solemnity is the shield of idiots.
[E.g.]