expression, noun
armchair theory; impractical idea; unworkable plan
Refers to a theory or plan that sounds good on paper but is unrealistic or cannot be implemented in practice. Often used critically.
彼の提案は机上の空論に過ぎない。
His proposal is nothing more than an armchair theory.
その計画は机上の空論
The plan turned out to be an impractical idea.