A facial expression where the eyes are rolled up, the mouth is open, and the tongue is sticking out. Ahegao is literally translated as "ahe face", where "ahe" (also "aheahe") is a sound effect for wild moaning.
This expression is usually (but not always) sexual. It can also indicate craziness or drug influence.
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.
source: danbooru wiki