To be Manichean is to follow the philosophy of Manichaeism, which is an old religion that breaks everything down into good or evil. It also means “duality,” so if your thinking is Manichean, you see things in black and white.
Manichean comes from the word Mani, which is the name of an apostle who lived in Mesopotamia in the 240’s, who taught a universal religion based on what we now call dualism. If you believe in the Manichean idea of dualism, you tend to look at things as having two sides that are opposed. To Manicheans, life can be divided neatly between good or evil, light or dark, or love and hate. When you see Manichean, think “two.”