Sorry, but it is perfectly acceptable in casual English-- it is far too common too ignore. 'Really' is of course just informal for 'very'. As for 'bad/badly', here is what the American Heritage Dictionary has to say: Bad is often used as an adverb in sentences such as The house was shaken up pretty bad or We need water bad.
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'Real bad', ' Real badly', 'really badly', 'really bad'...all mean the same! Real and bad can be used as both adjectives and adverbs. Real holds the same meaning as really, meaning very.
And bad, on the other hand, is both an adjective and an adverb.
Thus, 'real bad' or 'really badly' literally mean 'very badly'.
E.g.. I was hit real bad by a car.
OR I was hit really(ve