A variant spelling of aarrgh; an exclamation expressing strong frustration or anger.
An alternate spelling of aarrgh with an extra 'h' at the end, emphasizing the drawn-out nature of the sound. Since it's onomatopoeia, different writers spell it differently depending on how long or intense they want the sound to feel.
The fact that 'aarrghh' exists in dictionaries shows how flexible English is—the more 'g's and 'h's you add, the more frustrated the person sounds, so writers customize the spelling for dramatic effect!
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