An idiom meaning you can't stop people from talking; it's impossible to completely prevent gossip or speech.
ミスをしてしまったが、上司に説明しても人の口に戸は立てられず、既に噂が広まっているようだ。
I made a mistake; even after explaining to my boss, you can't stop people talking—it seems the rumor has already spread.
有名人は特に人の口に戸は立てられず、プライベートが話題になることが多い。
Celebrities especially can't stop people from talking; their private lives often become topics.
問題が起きたら隠そうとしても、人の口に戸は立てられず真相が出てくることがある。
Even if you try to hide a problem, you can't stop people from talking and the truth may come out.