De mortuis nil nisi bonum

— abbreviated Nil nisi bonum — is a mortuary aphorism indicating that it is socially inappropriate for the living to speak ill of the dead who cannot defend or justify themselves.

The full Latin sentence is usually abbreviated into the phrase (De) Mortuis nihil nisi bonum, "Of the dead, [say] nothing but good.

The Latin version of the Greek mortuary phrase dates from the translation of the book by Diogenes Laërtius, by the humanist monk Ambrogio Traversari in 1443.

For example, in Hebrew, one might use אחרי מות קדושים אמור (Aḥare mot k'doshim emor), which may be translated into: "After the death, say 'they were holy'".

The expression is formed by names of three consecutive sedras in Leviticus: Acharei Mot, Kedoshim and Emor, and has been taken to mean that one should not speak ill of the dead.