That can be impugned; open to (verbal) attack, open to question.(adjective)
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Use "impugnable" in a sentence
"We, too, find these twisted, evil minds of suicide bombers impugnable (ph)."
"East, certainly in none of the great self-governing colonies with which we rank ourselves, is the position of white man _qua_ white man so high, his status so impugnable, as in South East Africa."
"The only impugnable matter in the deed lies, as has been said, in the number of cardinals so created at a batch."