Literally translated from Latin, it means "against the world". It has been used to describe maverick, 'lone-ranger' individuals who lack interest in culture and relative comparisons made by people that the individual deems 'ignorant' (which doesn't make them wrong, necessarily).
As mentioned in Brideshead Revisited (movie 2008), when Charles says he has no family and Sebastian replies "you have me, Sebastian and Charles, contra mundum".
The phenomenon of someone looking a term up when they don’t know what it means to make it look like they are of higher intelligence than they actually are.
Person 1: “You will fall victim to the Mundubu effect in 30 seconds.”
Oblivious Person: *looks up Mundubu effect in order to act like they’re smarter than they actually are*