pulsate, shudder
悸 centers on a sudden, involuntary bodily reaction: a strong heartbeat or a shudder of fear or excitement.
悸 combines 忄 (heart) with 季, which likely contributes the sound and may suggest a seasonal or cyclical rhythm, though the exact historical development is uncertain.
The heart radical 忄 beside 季 (season) suggests a heart that beats with the sudden, intense rhythm of a seasonal storm: a shudder or pulsation.
For キ, imagine your heart giving a sudden shudder when you see a key (キー) that unlocks a forgotten memory: key -> キ.
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