revenge is a dish best served cold
Pierre Ambroise Francois Choderlos de Laclos (1741–1803) is often credited with the phrase's invention, with his 1782 book Les Liaisons Dangereuses being said to contain the line "La vengeance est un plat qui se mange froid." One of the first known English-language appearances is Kind Hearts and Coronets (1949): "Revenge is a dish which people of taste prefer to eat cold." It most famously appeared in its exact form in Star Trek II: The Wrath of Khan: "Ah, Kirk, my old friend. Do you know of the old Klingon proverb that revenge is a dish best served cold? It is very cold in space."
- An expression that emotional detachment is ideal when taking revenge, as one is righting the wrongs that have been done to the doer.
See also 
- http://www.menweb.org/scottrev.htm credits it as a Sicilian proverb