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thing that should be (carefully) avoided; no-no; thing to definitely not do; something forbidden; taboo
運転中の居眠りは禁物だと肝に銘じている。
I keep in mind that dozing while driving is absolutely forbidden.
交渉では感情的になることは禁物である。
Getting emotional is taboo in negotiations.
この病気の患者には激しい運動は禁物だ。
Intense exercise is forbidden for patients with this illness.