noun
taboo; no-no; thing to avoid
Used to warn that something is strictly inadvisable or forbidden in a particular situation. Often appears in the pattern 〜は禁物だ.
油断は禁物だ。
Carelessness is a no-no.
風邪のときは無理は禁物です。
When you have a cold, overexertion is something to avoid.
タブー is a direct loanword for a social or cultural taboo, while 禁物 is a native expression for something that should be avoided in a specific context, often used in everyday warnings.
Compound of 禁 (prohibition) and 物 (thing). The exact historical derivation is uncertain, but the meaning 'thing to be avoided' is well established.