急速
きゅうそく · kyuusoku · JLPT N3 · na-adj
rapid
🚀 急速 = urgent + speed: things moving at urgent speed are rapid
急 (kyuu) means urgent or sudden, as in 急行 express train. 速 (soku) means speed, as in 速度 velocity. Together they mean progress that happens at urgent speed: rapid, swift growth or change.
急 — heart radical 心 + 及 (reach)
A semantic-phonetic compound: the heart radical 心 supplies the meaning (an emotional state) while 及 ('to reach, catch up') gives the sound. Originally it depicted a racing, anxious heart — hence the modern senses 'urgent,' 'hurried,' and 'impatient.'