eye of newt

Eye of newt is an archaic name for mustard seed, one of many folk names that made common herbs sound mystical in old apothecary lists and spell recipes.

The term is best known from the witches' brew in Shakespeare's *Macbeth*: Eye of newt and toe of frog, wool of bat and tongue of dog.

Modern usage is almost always a playful reference to that scene, evoking witchcraft or potion-making.