Something that is most definitely not "really cool".
"I'm doing something really cool."
"Oh yeah, whats that?"
"I'm adding the word 'headphone-malfunction' to urban dictionary."
"That's really cool."
Something or someone that is so cool the meaning is useless when applied to said person/thing. In times like these, one must use the correct Really Cool to verbalize the utter coolness of said holy object...person/thing.
Beatrice: Man, that Johnny sure is a jive cat.
Harriet: One might say he is... REALLY COOL.
Beatrice: No. BLASPHEMER! ROT IN HELL!!