what's your 20?

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.

This voice-communication alternative to "where are you?" derives from the police 10-codes, no longer used in most jurisdictions but nearly-standard in the U.S. from the 1940's until federal agencies discouraged them in the 21st century. "10-20" meant "What's your location?" or "My location is..." depending on the context. Ten-codes peaked in public awareness in the 1970's when the hit song "Convoy" brought citizens-band use of "10-4" to a period of widespread recognition.

Your position, the place you are

What you ask a person when you want to know where they're at sexually.

As in "What's your location?" Truckers ask other truckers this question in an attempt to triangulate smokey's location.