a 10-4
A 10-4 is like saying, "Got it," usually when someone is telling you stuff and you are trying not to fall asleep.
"10-4, I got it," he said as he was trying to keep his eyes open.
She said, "10-4, I heard that," even though she was on the phone in the bathroom.
He muttered, "10-4, I heard you, I just don’t care."