Chiasmus

Chiasmus is a literary term where words, grammar, or ideas are repeated in reverse order.

A good example of this is any dialogue of The Sphinx from the movie Mystery Men.

Famous examples of this term:

"Fair is foul and foul is fair." - The Witches, Macbeth, Shakespeare

"By the day frolic, and by the night dance." - Samuel Johnson

“Let us never negotiate out of fear, but let us never fear to negotiate.” - John F. Kennedy