Whenever you consume a vast plentitude of drugs and/or alcohol. It is a reference to Fear and Loathing in Las Vegas, the work of gonzo journalism by Hunter S. Thompson (and adapted into a movie starring Johnny Depp). It is usually followed by the name of either the city you're in or whatever location the binge took place.
Dude, Alex and I did 30 reds, a tap of acid each, 6 lines, and half an ounce of pot each last night! It was fuckin fear and loathing in san francisco, man