The phrase 'the writing on the wall' is an idiom that refers to a warning or sign of impending doom. It originates from the Biblical story of Daniel, in which a mysterious hand wrote a message on the wall of King Belshazzar's palace. The crossword solution for this phrase is 'graffiti, ' which is defined as writing or drawings scribbled, scratched, or sprayed illicitly on a wall or other surface in a public place.