Stands for: That's What The Fuck I Said.
Used when someone repeats something you said or when you are agreeing with someone who shares the same opinion as you.
Robert: If you ever waded into the Intelligent Design debate, you surely encountered a PhD biologist or chemist confidently tell you, "The very notion of looking at an organism and speculating on whether it is 'designed' is unscientific."
And yet, that's the main topic regarding Wuhan.