is not a phrase for conveying information. It’s a phrase for conveying vibe . It’s for those moments when a simple “he’s big” or “this is new” feels insufficient. You need the maji de seriousness, the grammatical rupture of dekain , and the baffling English tag new to capture the absurdity of existence.

Your friend shows you a brand new, oversized hoodie. Look them dead in the eye and say: “That’s very uchi no otouto maji de dekain new of you.” (They won’t understand. That’s the point.)

But dekain goes further—it nominalizes the adjective. It turns “huge” into a thing : the hugeness itself. So when the sister says “maji de dekain,” she’s saying “Seriously, [this situation of] hugeness,” leaving the listener hanging.