Noun: "(chiefly Japanese fiction) a comically exaggerated facial expression consisting of flushed cheeks, rolled-back eyes, and the tongue hanging out, intended to depict a person having an orgasm"edit
2017, Carolina Lindström, "'The power of characterization': A comparative analysis of the transformative works created by the English-language and Japanese-language fandoms of BBC Sherlock", thesis submitted to Stockholm University, page 26:
During the set of intercourse at the end of the story, Sherlock is also drawn featuring ahegao expressions, frequently used in hentai or pornographic works.
2019, "Personals", The Medium (Rutgers University), 6 March 2019, page A6:
Without anime I would never have been introduced to such thought provoking concepts as ahegao or 15 year old girls with boobs the size of basketballs.