Na-adjective (keiyodoshi), Noun
normally being cold but at some prompt suddenly becoming lovestruck; hot-cold personality type
そうやって俺のことを気にしつつ、しかし素直になりきれない、そのツンデレさが今若い男の間で大ブーム！That way you keep thinking about me but are unable to admit to your feelings, that hot and cold feel is a real boom among young men!
Also written つんでれ