where does what’s your 20 come from

“What’s your 20?” comes from a system of radio codes called 10-codes [Dictionary of Slang by Dictionary.com]. These codes developed in the late 1930s when police squads began using two-way radios to communicate.

Originally, the complete code was “10-20,” which meant “location.” Over time, for quicker communication, the “10” part was dropped, leaving just “What’s your 20?” as a way to ask someone for their whereabouts.

This term was originally used by police and emergency services, then later adopted by CB radio enthusiasts and truckers for their citizen band radios. The use of 10-codes has declined somewhat in recent years, but “What’s your 20?” is still a recognizable phrase in popular culture.