Limewash is a type of white paint which has been used in many parts of the world since ancient times, and is still used today. In warm climates, the white color reflects heat and helps keep buildings cool. It is made from either powdered calcium carbonate (limestone) or slaked lime (calcium hydroxide, often in the form of lime putty) mixed with water. Limewash normally has a gleaming white appearance, however sometimes pigments are added to give other colors, such as beige, orange, pink, or light brown.
Tree trunks, especially fruit trees, are sometimes whitewashed to protect the tree from damage from sun and insects.
In English, the phrase "to whitewash something" has come to mean "to cover up an ugly truth". This is probably because when limewash is applied to a surface that is rough, dirty, or worn the imperfections are covered up, leaving a smooth, gleaming white appearance.