Ahegao (アヘ顔, lit. "ahe face", where "ahe" is a sound effect for wild moaning) is a facial expression where the eyes are rolled up, the mouth is open, and the tongue is sticking out. Frequently accompanied by blushing, drooling, sweating, raising the eyebrows, or a double V gesture.
Not to be confused with torogao, which is an expression that usually indicates a state of trance-like pleasure. Ahegao is a more wild expression.