so cool, n. — Green’s Dictionary of Slang

so cool n.

[cool adj. (1); a play on the area’s self-image]

(US campus) southern California.

1989		P. Munro Sl. U.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 Besides the geographic ‘So Cool’ meaning ‘Southern California’ and ‘No Cool’ meaning ‘Northern California’ there is a great number of euphemistic expressions which are about the ‘in’ things in California.