Ten-codes are signals used by radio operators and police officers to convey information using the number ten and then another number following it (hence the name). For example, 10-18 was assigned to mean "urgent", and 10-42 meant an officer was going off duty. Some of these phrases seeped into pop culture over time, especially due to cop shows' increasing popularity. What's your twenty to mean "where are you" emerged due to 10-20 being the code asking for location, and 10-4 is commonly used to mean "affirmative" or "roger", which is the same in the ten-codes (this is sometimes suffixed by good buddy, which was common slang among ham radio enthusiasts). All forms of those phrases peaked in usage sometime between the sixties and the eighties, but they're definitely around and kicking.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.