A fabrication or pipe dream: an unrealistic story or fantasy, often a plan unlikely to be realized; used often with a critical or dismissive tone.
そんな絵空事を言っていないで、現実的な計画を立てよう。
Stop talking about such pipe dreams and make a realistic plan.
彼の話は絵空事に聞こえたが、実は深い理由があった。
His story sounded like a fantasy, but in fact there was a deeper reason.
起業当初のアイデアは絵空事だと言われたが、努力で形にした。
At first the startup idea was called a pipe dream, but they shaped it through effort.