I happened to be researching early references to the colloquial “Frisco” as a shorthand for San Francisco. Noticed that Etymonline’s entry says “attested from 1856.”
The origins go back further, at least as far as 1850, as one can see by looking at these newspaper clippings:
Daily Placer Times and Transcript, Volume 1, Number 43, 9 March 1850
From context, the term seems to have been common currency by that date.
Hope that’s helpful.