To bellow is to shout in a deep, angry voice. If you constantly bellow at other drivers on the road, consider signing up for anger management classes.
Bellow comes from Middle English, and means "to roar like a bull." A cow's deep moo is called a low, so bellow just adds a big angry bull to the standard moo. We use bellow for human speech that has that angry power of a loud bovine, or for any loud, threatening noise. It can be a noun, as in the bellow of the thundercloud, or a verb, as when you bellow at your kid brother.