The Three Witches chant this line as they vanish into the fog in the opening scene of Macbeth, setting the moral weather for the entire play. Nothing in Macbeth will be what it seems: a loyal thane will become a tyrant, a brave soldier a coward, a trusted friend a ghost.
Shakespeare uses the reversal to unsettle the audience from the very first minute. By swapping fair and foul, the witches announce that categories have collapsed. Critics have called it the smallest chaismus in English literature that does the most work.
The line also rhymes with Macbeth's first spoken words later in the play, linking him to the witches before he ever meets them. That kind of long-range echo is part of why this remains one of the most studied reversals in Shakespeare scholarship.