1. To make use of or speak (to someone) on the telephone. Don't disturb me, kids, I'm on the horn to your father at the moment. After our dreadful meal that night, I was on the horn to the manager the very next morning.

2. dated To become lustful or sexually excited, especially of a man; to have an erection. Sarah became quite embarrassed when she noticed that her date was getting on the horn in the middle of the movie.