Meaning of “OtonaGai(大人買い)”
“OtonaGai(大人買い)” means buying a large amount of items (bulk purchases that children cannot do).
“Otona(大人)” means adults, “Gai(買い)” means buying.
For example, the act of making purchases, such as buying an entire manga series at once, which was impossible during childhood, is referred to as “OtonaGai(大人買い)”.
Example Conversation
A: I’m into in Chiikawa now, so went ahead and OtonaGai(大人買い) the entire series!
B: Oh, nice! How much did it cost?
A: I got the set with all the bonuses, so just under 20,000 yen, I think.
B: Oh, you’re so grown-up!
A: ちいかわにハマっちゃって全巻大人買いしてやった!
B: おー、いいねぇ、いくらくらいしたの?
A: 全巻特典付きの方を買ったから2万円弱かなぁ
B: おっとなー