The word comes from the Anglo-Saxon word for fate. Weird in this context means controlling human destiny and was spelled 'wyrd'. Whether or not Macbeth has the ability to shape his own destiny is a constant theme in the play, and the Witches are a symbol of this.

What was the meaning of wyrd?

Wyrd is a concept in Anglo-Saxon culture roughly corresponding to fate or personal destiny. The word is ancestral to Modern English weird, which retains its original meaning only dialectically.

Why are the witches called the weird sisters?

The sisters are called "witches" only once in the play—but they're called "weird" six. The word "weird" comes from the Old English term "wyrd," meaning "fate," so we're betting that they're in some way associated with the three fates of classical mythology.