A term used when ordering a tasty beverage with only one ice cube, or "rock."
A whole bunch of "rocks" can water down a top shelf drink, drown the flavor and waste your money. So instead of getting it "on the rocks" you can get it "Alcatraz" and show some respect for you booze.
Customer: Bartender, I'll have a Johnny Blue, Alcatraz.
Bartender: You mean on the rocks?
Customer: No, I mean one rock. If want water I'll order water.