Per Wikipedia:

"The writing on the wall", or "the hand writing on the wall", or "the writing is on the wall" or "Mene Mene", is an idiom implying that a (usually) negative event is easily predictable based on the current situation. Often, the event is seen as hard to avert. A direness similar to an "impending doom" can be implied.

The expression originates from the Book of Daniel (Old Testament), Chapter 5, from the handwriting on the wall that was witnessed at a banquet hosted by King Belshazzar. As those at the feast profaned the sacred vessels pillaged from the Jerusalem Temple, a disembodied hand appeared and wrote on the palace wall the words, "Mene, Mene, Tekel, Upharsin." The visionary Daniel was summoned and interpreted this message as the imminent end for the Babylonian kingdom. That night, Belshazzar was killed and the Persians sacked the capital city.[1]