person who spends a lot of time in their study; armchair expert
Often carries a slightly negative nuance of someone who is bookish and out of touch with the real world, similar to 'armchair expert'.
He is an armchair expert with almost no real-world experience.
Scholars of the armchair type sometimes don't listen to voices from the field.
理論家 is a theorist, which can be neutral or positive, whereas 書斎人 emphasizes the person's detachment from practical reality.
Compound of 書斎 (study, library) and 人 (person). The exact historical derivation is uncertain, but the word likely emerged in modern Japanese to describe a person who is always in their study.