Good point. It's actually unpossible.
> (source)
>
> While supposed to be transcription of a gasp after uttering , its impossibility of vocalization, that a follows a , has made it viral as an internet meme, eventually becoming a popular way to transcribe sound in (typically the end of) a phrase with a homosexual innuendo.
And the article is also kind enough to provide a pronunciation guide:
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> "aargh!"ɑːr
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> Although grammatically unpronounceable, it is usually read out as if , or those who enjoy this word sometimes claim that only (the meme origin) can pronounce it.
> In English, "aargh!"ɑːrmight be the closest pronunciation judging from its usage.