The phrase essentially means, "What is your location?" or "Identify your position," but is a corrupted phrase from the original "10-20" used by
United States law enforcement to verbally encode their radio transmissions to that non-police listeners would not easily discover police
operations, as well as to
communicate quicker and more efficiently by standardizing frequently used phrases.
These verbally-coded messages were called "10 codes", of which "10-20" stood for "Identify your position," or "Where are you?" originally. Other such codes include "10-7" meaning the officer was busy such as with a traffic pull-over, "10-8" meaning that the officer was back on patrol such as from having just written a citation, the popular "10-4" as an affirmative, "10-10" as a negative and "10-22" to disregard a previous transmission have only seen light integration into common use. It was not uncommon for a city to have its own set of
particular 10-codes for other phrases frequently used particular to that locale.
This code-phrasing is similar in design to Amateur Radio Operators' (which require an FCC license) use of Q-signals, such as QTH ("What is your location") and QSL ("affirmative/
understood") used to reduce the time needed to transmit and interpret a Morse-code transmission.
