prickly heat, rash
疿 centers on skin irritation from heat: prickly heat or a rash caused by blocked sweat glands.
疿 combines 疒 (sickness) with 弗, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a skin ailment.
The sickness enclosure 疒 wraps around 弗, which looks like a tangled web of arrows. Picture heat rash as tiny inflamed arrows pricking the skin under a blanket of sickness.
For ヒ, imagine the rash making you say "hee!" in discomfort: hee -> ヒ, as the prickly heat stings your skin.