heat rash, prickly heat, ulcers
痱 centers on skin eruptions caused by heat or irritation: heat rash, prickly heat, and by extension, ulcer-like sores.
痱 combines the sickness radical 疒 with 非, which likely contributes a phonetic element. The exact historical development is uncertain.
The sickness radical 疒 wraps around 非, which looks like a pair of wings or a barred gate. Imagine a feverish person whose skin breaks out in a rash that spreads like wings across their back.
For ヒ, picture the heat rash making you say 'hee!' in discomfort: hee -> ヒ, as you scratch the prickly spots.
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