1. a quick way to let a friend know if police are in the area, so he doesn't take a hit or empty the pipe out the window.
2. a great way to scare the shit out of a friend whos hitting the pipe in plain view or looking at a nug above the door/window boundary.
Whether the sayer of this warning is serious or not as to the presence of police is determined by how many times the phrase is said.
1. If said only once at reasonable volume, there are really police and u should check yourself.
2. If said multiple times at high volumes, chances are we're just messing around. (See example)
1. Real: "cops, chill" ("yo, po-po...")
2. Joking: "oh SHIT COPS CHILL, DUDE CHILL, COPS.. shit dude... cops chill."
*friend shits pants and hides everything under the seat*