In CB radio usage, "10-4" meant that a message had been acknowledged.

Format Web Page

Location PART 2 - FOUR HIDDEN CHIMNEYS CHAPTER 7: JUVENILE DELINQUENCY The Source Awakens

"In CB radio usage, “10-4” meant that a message had been acknowledged."

The Smithsonian explicitly glosses "10-4" as "message acknowledged", making the slang example safe. + Smithsonian object record dates the CB radio T-shirt circa 1975.

View Source

Source title CB radio tee-shirt

Source authors National Museum of American History

Source year 1975

Confidence high

Importance low