急に
きゅうに · kyuuni · JLPT N3 · adverb
suddenly
⚡ 急 = urgent, hurry; kyuu-ni sounds like on cue: it happens all at once
急 means urgent or steep, as in 急ぐ (isogu, to hurry) and 救急車 (kyuukyuusha, ambulance). Add に to make an adverb: urgently, so suddenly. The kyuu sound works as a hook too: something that comes in on cue arrives all at once.
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.'