Aaugh! is a phrase used when Peanuts characters scream. Many times they yell, facing up with their mouths almost as big as their faces.
The voice track of Charlie Brown (voiced by Peter Robbins) screaming "Aaugh!", when he failed the spelling bee in A Boy Named Charlie Brown, was used again in later specials even when the character was voiced by another actor. It was even used when other characters, especially girls, screamed.
Similarly, the screaming pose is also used whenever characters are shouting, laughing, or crying.