What is the origin of the phrase 'writing on the wall'?

It is from the Tanakh, also known as the Old Testament. Book of Daniel, chapter 5. It’s a metaphor meaning: you are about to crash and burn.

For people who believe the whole Bible is literally true, Daniel is history. For other people, which includes many devout Chrstians and Jews, it is historical fiction, interesting stories based on the historical kings of 6th century BCE Babylon.

In chapter 5, King Belshazzar is partying down and calls for the butler to bring the gold and silver goblets that Nebuchadnezzar II took from the temple in Jerusalem. The partiers drink from them, praising the gods of gold and silver, plus the gods of bronze, iron, wood and stone for good measure. (verses 1–4)

Alakazam! A disembodied hand appears out of thin air and writes on the plaster wall near the lampstand: “Many, many tickle a parson.” The king freaks out, as would I. (verses 5–6)

The king calls his counselors, but they are all “WTF?” The king says, “Useless bozos! Whoever can explain this spooky writing gets a purple robe, a gold chain, and the kingdom’s #3 job.” (verse 7)

Like Arthur pulling the sword from the stone, everyone strikes out until the queen reminds the king to ask Daniel, the Jew whom Nebuchadnezzar had appointed chief of the magicians, enchanters, astrologers and diviners. Belshazzar calls him in and offers him the reward. (verses 8–16)

Daniel says, “Keep your rewards, but I will answer anyway. (verse 17)

“Sucks to be you. Just as God humbled the arrogance of Nebuchadnezzar, making him crazy enough to live with wild donkeys and eat grass, he is going to humble you. The words are: mene, mene, tekel, upharsin. They mean:

Mene: God has numbered the days of your reign and brought it to an end.

Mene: God has still numbered the days of your reign and brought it to an end.

Tekel: You have been weighed on the scales and found wanting.

Upharsin: Your kingdom is divided and given to the Medes and Persians. (verses 18–28)

Belshazzar gave him the purple robe, gold chain and cushy job as promised. Which did no good, because Belshazzar died that same night, and Darius the Persian conquered Babylon (verses 29–30)

Daniel goes on to have even more improbable adventures. It’s a good read.