‘Grimalkin’ is an old word for a domestic cat, especially an elderly female one. It was used often during the 15th-18th centuries, when women tried as witches were accused of having cats as their familiars. Even Shakespeare used it in Macbeth, when one of the three witches refers to her familiar, who takes the form of a cat, as ‘Graymalkin’. SourceSource 2Source 3