Something that should be strictly avoided; a taboo or a definite 'no-no'.
大事な面接の前に夜更かしするのは禁物だ。
Staying up late before an important interview is a definite no-no.
お金の話を初対面の人とはするのが禁物だとは限らないが、注意は必要だ。
Talking about money with someone you just met isn't always taboo, but you should be careful.
その薬をアルコールと一緒に飲むのは禁物だ。
Taking that medicine with alcohol is something to be strictly avoided.