IPA: /əˈpjuːɡnər/
KK: /ɑˈpjuːɡnər/
A person who opposes or argues against something, often in a formal context.
The oppugner presented strong arguments against the proposed law.
Oppugner is derived from "op-" (meaning against) and "pugnare" (meaning to fight). The word describes someone who fights against or opposes something.
Think of someone who is 'fighting against' ('op-') something — that's what an oppugner does.