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.
A: What's taking so long?
B: I'm at a red light that won't turn green even though there's no cross-traffic.
A: What's your 20?
B: Avenue F and Kingston.
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.
The raw rookie cop had noticed that most police on duty substituted a different phrase rather than using the ten-codes he'd been taught in the Academy. He transmitted "where's your 20?" to his partner because he'd forgotten other cops usually said "what's your 20?". His veteran partner responded, "In my wallet with my five and two ones".
The nickname of White-Trash women who swear with every third word they say, find the time to smoke 2 packs a day at work, have a cell phone that was purchased in 1997, and who look like Anne Ramsey from "The Goonies."
Me: We're having some good weather today, aren't we?
What's Your 20?: Weather better fuckin stay nice all fuckin day cuz once I leave this fuckin shit hole I gotta pick up my bitch daughter and her fuckin cunt kids.
Me: (Awkward smile and nod)
Whats your 20?: I tell ya, I fuckin ate some bean-picker food last night and, fuck, I've been shitting every 10 minutes since. The fuckin women's toilet looks like a fuckin diarrhea-shit grenade fuckin went off in there. Any chance I can bum a fuckin smoke off ya?
Me: (Blank stare)
word of the day: January 20, 2012
Your position, the place you are
Johnny,i'm going to the party , what's your 20?
My 20 is Baker Street.