〜が最後／〜たら最後

Meaning

States that once one does it, an irreversible bad result follows. Expresses the strong assertion that once it is set off, an undesirable situation is sure to ensue.

Examples

彼は一度寝たが最後、地震が来ても目を覚まさない。 Once he falls asleep, he will not wake up even if an earthquake strikes.