grimalkin

In the opening scene of Macbeth, one of the three witches planning to meet with Macbeth suddenly announces, "I come, Graymalkin." The witch is responding to the summons of her familiar, or guardian spirit, which is embodied in the form of a cat. Shakespeare's "graymalkin" literally means "gray cat." The "gray" is of course the color; the "malkin" was a nickname for Matilda or Maud that came to be used in dialect as a general name for a cat (and sometimes a hare). By the 1630s, "graymalkin" had been altered to the modern spelling "grimalkin."