OK; all right. Used to indicate affirmation or acknowledgment. The phrase comes from CB radio, which was commonly used (especially by truck drivers) before the proliferation of cell phones. A: "I'm going to pull over up ahead." B: "10-4."
10-4
verb
See ten-four
ten-four
and 10-4
interj. okay. (Citizens band radio.) Ten-four, old buddy. I will do that.