Definition of ironclad adjective from the Oxford Advanced Learner's Dictionary

 

ironclad

 adjective
adjective
BrE BrE//ˈaɪənklæd//
 
; NAmE NAmE//ˈaɪərnklæd//
 
 
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so strong that it cannot be challenged or changed an ironclad alibi/contract/excuse/guarantee His memo is ironclad proof he was involved. compare cast-iron (2)
See the Oxford Advanced American Dictionary entry: ironclad