DAVINA: The witches refer to themselves as the "Weird Sisters", which is also how Macbeth and Banquo will refer to them. 

RALPH: The word weird derives from the old German verb wurt, meaning to turn into, to become. In Shakespeare’s day, to say something was weird, meant that it had the supernatural power to cause something to happen, or to know that something will happen. 

DAVINA: It’s only in the 19th-century, that weird has taken on its modern sense of referring to anything strange or mysterious. 

RALPH: The phrase "Weird Sisters" in fact had a very specific meaning. It referred to the three ancient mythological goddesses of Destiny, the three Fates, who were often depicted with a spinning wheel. The first Fate is spinning out the thread of life, the second is measuring its length, and the third is cutting it off at the moment of death. 

DAVINA: By referring to these characters as the "Weird Sisters”, Shakespeare seems to be giving them a symbolic and allegorical quality. This was a common practice in the theater of Shakespeare’s time, to have figures on the stage who were not part of the world of the play, but could be used to comment on the action.