暢気（のんき） describes someone who is carefree, unconcerned, or sometimes a little oblivious to the situation around them. The kanji spelling is less common and more visually distinctive than the usual hiragana のんき.
In the song, the narrator says 相変わらず暢気ね: “You’re as carefree as ever.” Despite the painful farewell, she affectionately teases the other person for seeming relaxed, revealing that she still loves this part of them.
This is a breakup song sung from inside the breakup, by someone who keeps almost saying what she wants and then taking it back. The first image is the best one: the goodbye gets stuck in her throat, so she says "ありがとうって言ったの" (I said "thank you") the way you'd cough to clear it. She digs through her pocket for the next thing to say and finds only "the me who loves you". Then the chorus: "青いまま枯れてゆく" (withering while still green). It's a leaf or a flower dying before it ever ripened, and she picks it up, thinks it looks like her, and crushes it in her hand along with the feelings she'd kept at the back.
Every chorus ends the same way. She asks him to hold her right now, to say he needs nothing as long as she's there, to kiss her, and then: "なんてね 嘘だよ" (just kidding, that was a lie). なんてね is a little phrase Japanese speakers tack on to take back something they just said, like "...or not." Watch what follows it each time: first "ごめんね" (sorry), then "元気でいてね" (take care of yourself), and at the very end "さよなら" (goodbye). So the goodbye that was stuck in her throat in the first line finally gets out in the last one.
The middle is full of small, specific memories, and that's where it hurts most. She remembers him saying "一人にしないよ" (I won't leave you alone) and admits she was secretly happy about it. He looks relieved that she isn't crying, and she calls him "暢気" (easygoing, a bit oblivious) and says she loves that about him too. Nice word to learn, by the way: 暢気 (nonki) is the gently teasing way to call someone carefree. The title means "happy ending", which is either very sad or very kind, depending on how much you believe her when she says she's fine.
back number is a Japanese rock trio formed in Gunma Prefecture in 2004 by singer and guitarist Iyori Shimizu, with Kazuya Kojima on bass and Hisashi Kurihara on drums.
The band signed to Universal Sigma and made its major-label debut in April 2011 with the single "Hanabira".