This word comes directly from the Latin “hirrient,” which comes from the verb “hirriō,” meaning “to snarl.” However, the adjective in English doesn’t refer to an angry snarl. Instead, it’s more like a cat purring, and in phonology it describes certain words or sounds that humans make. Those sounds are more commonly described as “trilled.” For instance, the rapid vibration of the tongue or uvula that produces the hirrient rolling “r” in some languages is called an alveolar trill.