English version

a cast-iron excuse/alibi/guarantee etc

From Longman Dictionary of Contemporary Englisha cast-iron excuse/alibi/guarantee etca cast-iron excuse/alibi/guarantee etcEFFECTIVEan excuse etc that is very certain and cannot fail cast-iron
Examples from the Corpus
a cast-iron excuse/alibi/guarantee etcAccording to Ken Harris, she has a cast-iron alibi.Do not expect a cast-iron guarantee of success.
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Word of the day atypical not typical or usual