PLATITUDE. An idea (a) that is admitted to be true by everyone, and (b) that is not true.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Book of Burlesques, “The Jazz Webster” (1924)
    (Source)

See his definition of "epigram."

Variant:

Platitude — An idea (a) that is admitted to be true by everyone, and (b) that is not true.
[Chrestomathy, ch. 30 "Sententiae" (1949)]