This means that if you do something, a bad thing will definitely happen. You cannot go back once you do it.
Verb [た-form] + が最後Primary
This means that if you do something, a bad thing will definitely happen. You cannot go back once you do it.
一度口に出したが最後、もう後には引けないぞ。
Once you say it out loud, it's final; you can't take it back.
あの食いしん坊の弟にケーキを見られたが最後、全部食べられてしまうよ。
Once my gluttonous little brother sees that cake, it's over; he'll eat all of it.
彼は一度決心したが最後、誰が何を言っても考えを変えることはなかった。
Once he made up his mind, that was it; he would never change his mind no matter what anyone said.
このコンピューターウイルスは、一度感染したが最後、全てのデータが破壊されてしまう。
Once this computer virus infects a system, it's the end; all data will be destroyed.
うちの猫は、外に出たが最後、なかなか家に戻ってこない。
Once our cat goes outside, that's it; it doesn't come back home for a long time.
This phrase is like a warning sign. It tells you that once something happens, you cannot go back. The result will be bad, and you cannot change it.
This is like "たら最後" but more formal. It means something bad will definitely happen after an action. "〜ようものなら" is different. It talks about what might happen if you try something. This phrase talks about what happens after you have already done something.
Don't use this when something good happens. For example, you can't say "Once I ate the cake, I was happy." You must use the past form of a verb. The next part always talks about a bad thing. This bad thing happens because of the first action. It is the only reason for the bad thing.
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