What it means
が最後 typically follows the plain past form of a verb and means that once the stated action occurs, an irreversible or unavoidable consequence follows. It often conveys a strong warning that there will be no opportunity to undo the action or escape its result. The pattern is common in serious, written, and formal contexts, although it can also appear in careful speech.
Usage note
が最後 is not a neutral way to say simply “after” or “once.” The following result is usually negative, difficult to avoid, or impossible to reverse, and the construction often has an emphatic or cautionary nuance.
How to use it
一度その契約書に署名したが最後、条件を変更できなくなります。
Once you sign that contract, you will no longer be able to change the terms.
一度その契約書に署名したが最後、条件を変更できなくなる。
Once you sign that contract, you will no longer be able to change the terms.