A proverb meaning 'dead men tell no tales'—the dead cannot speak in their defense, so the truth may remain unknown or accusations cannot be answered.
現場にいた人が亡くなってしまったため、死人に口なしで真相が不明だ。
Because the person who was at the scene died, it's 'dead men tell no tales' and the truth is unclear.
証言する人物が亡くなったので、死人に口なしと言わざるを得ない。
The witness has died, so one has to say 'dead men tell no tales.'