Sardonically lays out, sneering, what's impossible to contradict (8)
I believe the answer is:
ironclad
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for ironclad that I've seen before include "Old armoured warship" , "Vessel well protected" , "Protected warship" , "Old cairn with metal protection" , "monitor, for example" .)