That depends whether you mean 'cool' as in 'cold' or 'cool' as in 'fashionable'. I imagine that the Spanish might use different words for those two meanings.
'edit' I think it would be "Quedate chido"
I am cool - soy fresco
soy cool
fresco/a
fresco/a
you say: "puedes calmarte?"
Es chido.
it's a slang way to say: stay cool :)
!chido nombre!
Aguas frescos.
"Genial" or "chido"
quedarse en
fresco, tibio = cool (adjective) refrescar = to cool