I'm thinking of skinny shaming a co-worker, she annoys me a lot
Would it upset your co-worker if you skinny shame them?
It probably will, but I don't care about that bitch.
You shouldn't insult a co-workers appearance. They'll become angry and could report you to HR. If HR backs them, then you could be reprimanded or even fired.
Whatever, I bet I will get away with it.
I think it would be better to assume that you would get caught, so you won't have to even think of doing something like this. How do you think she would feel being made fun of like that? A workplace is supposed to be a safe space and you can contribute to that by making her feel welcome instead.