pre ‧ pos ‧ ter ‧ ous /prɪˈpɒst ə rəs $ -ˈpɑːs-/ BrE AmE adjective formal
[ Date: 1500-1600 ; Language: Latin ; Origin: praeposterus 'with the back part in front' , from prae- ( ⇨ ↑ pre- ) + posterus ( , ↑ posterior 1 ) ]
completely unreasonable or silly SYN absurd :
The whole idea sounds absolutely preposterous!
—preposterously adverb
—preposterousness noun [uncountable]