In standard Japanese, yarou means bastard, son of a bitch, and asshole.

Commonly used in the phrase "baka yarou" = stupid bastard.

Also, phrase made famous by Antonio Inoki is "Nanda kono yarou!" = loosely translated as "what the fuck, you bastard". As a matter of fact, on my first trip to Osaka to see my friends from college, I bought an Antonio Inoki t-shirt at America-mura in Shinsaibashi with this written on it. I have since lost that shirt though. :(

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However, in the kansai dialect, yarou has the same meaning as darou in the tokyo dialect = isn't it? aren't you?. Thus, it can be sometimes confused. When used as a tsukkomi after someone does or says something stupid, aho yarou means "you are stupid, aren't you.