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2.
Saying used by small minded people after absolutely EVERYTHING you say, even when what you say is totally NOT cool.
Dude: Hey babe, wanna go get a beer tonight?

Babe: Can't, I'm not feeling well.

Dude: That's cool.
by enchantedm August 03, 2005
 
1.
Most often means something is of interest. It can also mean however, that something is not interesting at all.
"Did you hear Brandy was coming in late?"

"That's cool......"
by That's cool September 04, 2009
 
3.
Much like heh, it's what people say when they are uninterested and/or bored by what you're talking about.

Can be shortened to just "cool...".
Girl: And this weekend the girls and we went shopping, got our pedicures done, and saw a movie!
Guy: uh-huh... that's cool...
by illEATurHARTout May 23, 2005