He lies like an epitaph.

The saying accuses someone of dishonesty by comparing his words to an epitaph that praises the dead more generously than the facts warrant.

Its sting lies in treating respectful memorial praise as a familiar kind of falsehood. The comparison makes the accusation pointed and darkly humorous.

Use it as a sharp, wry response to someone whose account seems conspicuously untrue or flattering. Its reference to memorial inscriptions gives the remark a macabre edge, so it may sound harsh in a solemn setting.