Which bit do you mean welbeck? She really knows what it means...and pretends she doesn’t, or she was never called it at all ? I looked it up?. Horrible....if that’s what he means. How old is he?
It sounds as if you suspect that it might be uncomplimentary. Is he a teenager at the really awkward stage?
Either way I would find out and con'front him or tell his parents. Using foul language to ones grandmother, because she does not understand what it means is just about as contemptible as it comes. Deal with it upfront and make your anger at his despicable behaviour clear.
How old is he? I don’t know what it means either, but depending on his age, could mean totally different things. It might be something he’s heard, and has just regurgitated.
As long as he didn`t put the word "out" on the end,i wouldn`t worry. I don`t know where the English language has gone. It`s changed beyond belief. Wicked was bad once,now it`s good,chips was a food,now it`s in a device etc.