A phrase meaning 'do not tell others' or 'keep this secret' — a warning not to disclose information.
封筒に「他言無用」と書いて渡した。
I handed it over with '他言無用' written on the envelope.
重要な話だから、他言無用でお願いします。
This is important information, so please don't tell anyone.
会議の内容は社外秘のため、参加者に他言無用と伝えた。
Because the meeting content was confidential, participants were told not to disclose it.