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

 

unimpeachable

 adjective
adjective
BrE BrE//ˌʌnɪmˈpiːtʃəbl//
 
; NAmE NAmE//ˌʌnɪmˈpiːtʃəbl//
 
(formal, approving)
 
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that you cannot doubt or question evidence from an unimpeachable source
See the Oxford Advanced American Dictionary entry: unimpeachable