Tooting a horn is an action that has been used for centuries to signal a warning or alert. It is often used in military and emergency situations, as well as in celebrations and ceremonies. The term 'sound a horn' is an idiom that means to make a loud noise with a horn or other instrument. The crossword solution for this term is 'TOOT, ' which is a verb meaning to make a short, sharp sound with a horn or other instrument.