慶応大学教授のCharles De Wolf氏が<A href="http://blog.alc.co.jp/blog/3302827/86243">ASJのSupplement, Life and Learning</A>に寄稿した文の中に "I was then as snobbish an armchair socialist as any,..." という記述がありました。
早速英英辞典でarmchairを引くと、armchair traveler, CRITIC etc.: someone who talks or reads about being a traveler, CRITIC etc., but does not have any real experience of doing itと説明がありました。つまり、実際の経験に基づかない観念的、空論、机上の理論を振り回す人のことですね。では日本語で言う机上理論はarmchair theoryでよさそうだと見当がつきます。 実際にarmchair theoryを使った例文をBBC.co.uk から引用します。
From a scientific standpoint, and excluding the political issues for now as this is where all the armchair theory comes in, there is NO CHOICE but to adopt Nuclear Power if humanity is to survive the next 100 years.