i have never heard that phrase. is it like when you smack a toddler’s diaper-padded behind and they start bawling, not because it hurt but bc their feelings were hurt?
The phrase has nothing to do with any body part/section.
No, it’s typically used of adults.
Susie was butthurt because Jenny wouldn’t loan her money.
It’s called butthurt because the person “suffering” from the condition, acts as though he/she literally (*ahem*) “took it” in the rear.