What is the X in X-mas?
1590s, in fresco, literally "in fresh," with a sense of "painted on fresh mortar or plaster," from Italian fresco "cool, fresh," from Proto-Germanic *friskaz (see fresh (adj.1)).
A painting on wet plaster. When the plaster dries, the painting is bonded to the wall. Fresco was a popular method for painting large murals during the Renaissance. The Last Supper, by Leonardo da Vinci, is a fresco, as are the paintings by Michelangelo in the Sistine Chapel.