1. Currently using or speaking (to someone) on the telephone. Jerry, get on the horn to the warehouse and tell them we need a few more pallets of paper.After our dreadful meal that night, I was on the horn to the manager the very next morning.
2. dated Lustful or sexually excited, especially of a man; having an erection. Sarah became quite embarrassed when she noticed that her date was on the horn in the middle of the movie.