That's Cool

That's Cool is a phrase people tend to use when they don't know what to say or are uninterested.

In this context, the word "cool" isn't necessarily positive, and becomes more neutral. This is why there is sometimes confusion when the phrase is used.

Person A: Hey man, did you hear I just got a new car?
Person B: That's cool.

OR

Person A: Did you know Nick broke his wrist?
Person B: That's cool.

That's Sick

That's sick can have different meanings, so you have to be conscious of the context and the way it's expressed.

1; Can be used to express awe or admiration for something interesting.

2; Can be used to express a disdain for something not-very-pleasant.

Person A: I got a finally got a new phone!
Person B: That's sick! You gonna give me your number?

OR

Person A: And he just gutted the fish! In front of me!
Person B: Ew, that's sick!