Oppugn is a verb that means to call into question the truth or validity of. 

Our word of the day combines the Latin prefix O-B which means ‘against,’ and the word pugnare (poog NAR ay) which means ‘to fight.’ To oppugn something means to oppose it, more specifically, to oppose the validity or truth of it. Example:

I didn’t mean to oppugn the judge's decision last week, but anybody watching that pie-eating contest could clearly see that I was the winner.