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 etc• According to Ken Harris, she has a cast-iron alibi.• Do not expect a cast-iron guarantee of success. 