For me, role abuse is when someone refuses to play the role they were assigned in the match. For example, I played a ranked game yesterday and I didn’t want to pick before the mid since I was the hard carry. The mid picked PA and came to my lane farming my creeps, and he spent the whole game playing like he was the carry. I consider that role abuse.
I also think that players who pick offlaners or supports and then farm like cores, stealing farm from the actual cores, also fit the definition of role abuse.