So, somebody (or some idea) sounds really smart. They use pedantry to impress stupid people. But you actually take 3 seconds to listen to what they're saying and, what do you know, it's the stupidest thing you've ever heard.
In case you haven't figured it out yet, this word comes from the fact that most of the ideas and people that this word applies to are found in colleges (it's what happens when you have someone hellbent on impressing their professors and friends who, sadly, lacks the capacity for forming a coherent thought).
Remember that time so-and-so was telling you how really the movie "Aladdin" is all about nihilism? Yeah. That guy was collegey.