I want to call a classmate with a speech impediment "Sid"
I don't understand. Why would you want to call a classmate with a speech impediment "Sid"?
I'm just making fun of that fucker for talking funny.
Making fun a classmate for their speech impediment is not a nice thing to do. I think you should try to have some sympathy for them, and not do anything to make them feel bad about their speech issue.
Fuck you, I'm going to call him "Sid".
I think that's a really bad idea. Not only will it likely make this person upset, it could make other people feel bad too. Making fun of this person would be a terrible thing to do, so I suggest you don't.