Somebody who lies about being more knowledgeable about something because they saw you talking about it. Usually by one upping you as a fan of an something. Short for Sophomaniacal Poser.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.