ordering a "water" at a fast food restaurant, only to fill it up with some other beverage of your choosing when no employees are looking. comes from the stereotype that jews are cheap, and would therefore partake in this activity.
mike: i'm hella thirsty, let's hit up captain d's man
tyrone: ok, shit, i only have a dollar, but i'm really thirsty too
mike: it's cool dude, let's just get jewish waters and then we'll have enough money to get food too!