熱心
ねっしん · nesshin · JLPT N4 · na-adj
enthusiasm
🔥❤️ 熱心 = hot + heart: a heart running hot is enthusiasm
熱 (netsu) means heat or fever, and 心 (shin) means heart or mind. Netsu + shin contracts to nesshin, with the small っ doubling the s. A hot heart is one that cares keenly, so the word covers enthusiasm, zeal and eagerness.
心 — pictograph of a heart with chambers and aorta
心 is a pictograph showing a stylized heart, with strokes representing the organ's chambers and the aorta at the top. Originally meaning the physical heart, it broadened to cover mind, feelings, and intentions, since ancient Chinese thought located thinking and emotion in the heart.