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2.
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!!
by Katron February 01, 2004
 
1.
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."
by Jeff June 10, 2006