A cop walking as opposed to driving. A cop driving is called "5.0" or "five-oh" whereas a cop walking is called a "9" or "nine." Apparently, the word "nine" is because typically intercity cops are issued a 9mm pistol, and the Highway Patrol cops used to drive
Mustang 5.0s, so if they where in their
car they'd be called "five-oh." However, this does not work as well when you assume (correctly so) that the phrase "five-oh" comes from the
police reference from the TV show "Hawaii Five-0".