You may know today’s word as a generalized term for anything unusual, but ‘weird’ also has older meanings that are more specific. ‘Weird’ derives from the Old English noun ‘wyrd,’ essentially meaning ‘fate.’
By the late 8th century, the plural ‘wyrde’ had begun to appear in texts as a gloss for ‘Parcae,’ the Latin name for the Fates — three goddesses who spun, measured, and cut the thread of life. In the 15th and 16th centuries, Scots authors employed ‘werd’ or ‘weird’ in the phrase ‘weird sisters’ to refer to the Fates.
William Shakespeare adopted this usage in Macbeth, in which the ‘weird sisters’ are depicted as three witches. Subsequent adjectival use of ‘weird’ grew out of a reinterpretation of the ‘weird’ in Shakespeare.