expression, godan verb (-tsu)
speak ill of the dead; criticize the dead
Idiomatic expression meaning to criticize or badmouth someone who is already dead and cannot defend themselves. Often carries a nuance of pointlessness or cruelty.
もう亡くなった人のことを悪く言うのは、死者に鞭打つようなものだ。
Speaking ill of someone who has already passed away is like flogging a dead horse.
The expression literally means 'to whip the dead'. The exact origin is uncertain, but it is a set phrase used to describe the act of criticizing someone who can no longer respond.