Frisco is the nickname that people who have never been to the bay area give to San Francisco.
Don't ever say "frisco" unless you want to be ironic or want to really annoy some native San Franciscans.
Them's fightin' words.
As Emperor Norton says:
Whoever after due and proper warning shall be heard to utter the abominable word "Frisco", which has no linguistic or other warrant, shall be deemed guilty of a High Misdemeanor, and shall pay into the Imperial Treasury as penalty the sum of twenty-five dollars. —1872