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Aaugh! is a phrase used when Peanuts characters scream out of frustration. Many times they yell, facing up with their mouths almost as big as their faces. The first appearance of the face is in the strip from June 13, 1958.
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.