The witches use the phrase "foul fair" to describe the paradoxical and deceptive nature of their prophecies. In Act 1, Scene 1, they address Macbeth as "fair is foul, and foul is fair" (l. 11). This statement sets the tone for the play by suggesting that appearances may be deceptive and that things may not always be as they seem. The witches' use of the phrase "foul fair" also foreshadows the many moral ambiguities and complexities that Macbeth will face throughout the play.
Macbeth echoes the witches' phrase "foul fair" when he confronts the ghost of Banquo in Act 3, Scene 4. Macbeth describes Banquo's ghost as "the foulest thing that mine eyes have looked on" (l. 115). This description suggests that Macbeth is haunted by his own guilt and by the dark deeds he has committed in order to obtain the throne. Macbeth's use of the phrase "foul fair" also reflects his confusion and internal turmoil as he struggles with the consequences of his actions.
The phrase "foul fair" thus becomes a central motif in the play, reflecting the paradoxical and deceptive nature of power, ambition, and the human condition. It foreshadows the ultimate downfall of Macbeth and suggests that even though he may achieve worldly success, he will ultimately pay a high price for his actions.