Noun: to summarize, one who sits around and acts like they're an expert on something, yet they've had no involvement or factual research of it.
A typical armchair philosopher is somebody who is a complete know-it-all, usually a douchebag or self-declared intellectual. They always feel the need to seem intellectually superior to others, by continuously arguing about any subject they see in media, conversations, etc. and quoting themselves as experts on the subject. Most conversations with armchair philosophers contain their opinions restated as knowledge, false or misheard/rumored "facts", and other idiocy. Especially noted is how armchair philosophers typically do nothing to prove their intelligence: they sit around on the sidelines and make judgements where careful analysis is needed. Topics can range anywhere from video games on a message board to the origins of the universe. They are also extremely prevalent on the Internet, where they can act douchey under the guise of anonymity. This is a common form of trolling.
Armchair philosophers are named after the old stereotype of old rich men in robes smoking pipes, sitting by the fire, pretending to know a thing about politics.
It is also notable, especially concerning the under-descriptive and generic language and baseless accusations, plus the long-winded description of armchair pholisophers, that this entire definition is composed of armchair philosophy.
Jake is such a damn armchair philosopher, acting like he's known that Senate election candidate his whole life. He's probably never even voted!