Frisco is the nickname that people who have never been to the bay area give to San Francisco.

You might hear this phrase uttered aloud by a tourist in Fisherman's Wharf, Union Square, or in a Cable Car.

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