10-4 means “message received.” It is the most famous phrase in CB radio slang.
Developed in the 1950s by truckers and emergency services, the “10-code” system was designed to keep communication brief over crowded radio channels. “10-1” meant bad signal. “10-20” meant location. “10-4” meant understood, affirmative, copy that. It was efficient. It was clear. It spread.
By the 1970s, CB radio had become a cultural phenomenon. Truckers used it to warn each other about speed traps. Hobbyists used it to chat across states. Hollywood noticed. Films like Smokey and the Bandit (1977) and Convoy (1978) made CB slang mainstream. Suddenly, everyone knew what a “bear” was (a police officer) and what “breaker one-nine” meant (a channel request).
“10-4” outlived the CB craze. Police still use it. Pilots use variations. Civilians say it without knowing where it came from. It is one of those phrases that entered the language so thoroughly that its origin became invisible.
The code is not universal. Different departments use different meanings. Some have moved away from 10-codes entirely after 9/11, when inter-agency confusion proved dangerous. But “10-4” persists. It is too short, too useful, and too cool to die.
10-4, good buddy. Copy that.