Taken from Danbooru
アヘ顔, where 顔 (_kao_) means face.
2ch/pixiv term for the weird face occasionally seen in H-games and much more in neta images. Canonically, ahegao has eyes rolled up, mouth open and tongue sticking out, like so: http://www.youtube.com/watch?v=S1FwGF4Y0Vk#t=02m05s
The English equivalent is usually fucked_silly.