I tar and feather featherweights until my 'fetti straight
“Tar and feather” is a sort of vigilante justice where someone who is presumed guilty has tar thrown on them and feathers thrown on the hot tar to embarrass the guilty.
A featherweight is a weight class in boxing/mixed martial arts known for its small fighters.
So Royce is saying he’s embarrassing the soft, under-belly of popular rap all the while making sure he makes his money.
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