Meaning of “Natsui(なつい)”

“Natsui(なつい)” is an abstraction for “Natsukashii(なつかしい)” which means nostalgic or memories, kind of.

Example Conversation

A: Look, look! Do you remember this?
B: Wow, I had that when I was in elementary school! So “Natsui(なつい)”!

A: みてみて！これ覚えてる？
B: うわー、小学生の時に持ってた！なついなぁ。