A verb defined as to "assail by criticism, argument or action" (dictionary.com). It is also used as a synonym of the word "dispute." to argue against something.
The media has substantiated enough footage to oppugn Hillary Clinton's claims about being in danger upon her arrival on her 1996 trip to Bosnia.
Husband: "Why do you oppugn my statements so much, honey?"
Wife: "Because you've been sleeping around with women behind my back, and I have the panties to prove it."