What is the verb form of horrible?

There is no verb form of "horrible". "Horrible" is an adjective describing something unpleasant or frightening.

To express the idea of "being horrible", you would use verbs like:

* To horrify: to fill with horror or dread.

* To terrify: to fill with terror or fear.

* To disgust: to cause a feeling of revulsion or aversion.

* To offend: to cause someone to feel hurt, angry, or upset.

For example:

* "The haunted house horrified me."

* "The clown terrified the children."

* "The smell of the garbage disgusted me."

* "His rude behavior offended everyone."