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Kelly Osbourne is jumping to the defense of her late father and his hometown ... after WWE star Becky Lynch cracked a crass joke about him on "Monday Night RAW."

Ozzy's daughter went off on IG in response to Becky's head-turning remarks ... calling her a "disrespectful dirtbag."

She continued unloading with ... "Birmingham would not piss on you if you were on fire."

Kelly then takes aim at WWE ... blasting the organization "for allowing such things to be said about my father and his home!!!"

ICYMI ... Becky and Nikki Bella were in a heated confrontation in the ring ahead of their upcoming Intercontinental Championship match this Sunday.

Lynch told a crowd of Birmingham fans she wouldn't do the match in their city ... going on to say, "The only good thing that came outta here di

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