The story
From Japanese アヘ顔(がお) (ahegao), from アヘアヘ (aheahe, “panting”) + 顔(かお) (kao, “face”).
Etymology from Wiktionary, CC BY-SA 4.0.
ahegao
Today it means: An exaggerated facial expression consisting of flushed cheeks, rolled-back eyes, and the tongue hanging out, intended to depict a person having an orgasm. Full definition →