spasms, (kokuji)
癪 centers on sudden, involuntary muscular contractions: spasms. It is a kokuji, a kanji created in Japan, and its meaning is tied to this specific physical reaction.
癪 is a Japanese kokuji that combines the sickness radical 疒 with 積, likely chosen for its sound しゃく to represent spasms. The exact historical reasoning is uncertain, but it follows a common pattern of using a phonetic component inside a semantic enclosure.
The sickness enclosure 疒 wraps around 積, which suggests something piling up. Picture spasms as a pile-up of sudden muscle contractions inside the body.
For しゃく, imagine a person having a spasm and shouting 'Shack!' as their muscles lock up: shack -> しゃく, and the sudden cry matches the spasm.
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annoyance; irritation; vexation
temper; irritability; fit of anger; tantrum (esp. of children); temper tantrum
impudent; cheeky
to lose one's temper; to throw a tantrum; to fly into a rage
hot-tempered person
to annoy; to irritate; to cause offence; to provoke; to get on someone's nerves
cause of annoyance; cause of irritation; irritant; peeve