Contracted from Belsharezar: from Bel, the Babylonian idol, and shar, a "king"; zar is a common Babylonian termination, as in Nebuchadnez-zar. His solemnly instructive history is graphically told in Daniel 5. See BABEL; BABYLON, for the remarkable confirmation of the Scripture account of his death on the night of revelry in the siege of Babylon; which is also stated by Xenophon; whereas Berosus in Josephus calls the last king Nabonedus (Nabonahit, i.e. Nebo makes prosperous) and says that in the 17th year of his reign Cyrus took Babylon, the king having retired to Borsippa (the Chaldaean sacred city of religion and science); and that having surrendered there, he had a principality assigned to him in Carmania by Cyrus. The inscription at Umqeer (Ur of the Chaldees), read by Sir H. Rawlinson, strews that Nabonedus admitted his son Belshazzar into a share of the kingdom, just as Nabopolassar admitted Nebuchadnezzar his sort to share in the government, Xerxes admitted his son Artaxerxes, and Augustus his successor Tiberius; so that the discrepancy is cleared.
Nabonedus, defeated by Cyrus in the field, fled to Borsippa, and survived. Belshazzar fell in the last assault of Babylon. Xenophon calls the last king of Babylon "impious," and illustrates his cruelty by the fact that he killed a courtier for having struck down the game in hunting before him, and unmanned Gadates a courtier at a banquet, because one of the king's courtiers praised him as handsome. His reckless infatuation is marked by his making a feast when the enemy was thundering at his gates; compare 1 Thessalonians 5:3-7 for the lesson to us. He set at nought eastern propriety by introducing women and even concubines at the feast. His crowning guilt, which made the cup overflow in vengeance, was his profaning the vessels of Jehovah's temple to be the instrument of revelry to himself, his princes, wives, and concubines, drinking out of them in honor of his idols.
Security, sensuality, and profanity are the sure forerunners of the sinner's doom. Intoxicating drinks tempt men to daring profanity, which even they would shrink from when sober. To mark the inseparable connection of sin and punishment, "the same hour" that witnessed his impious insult to Jehovah witnessed the mysterious hand of the unseen One writing his doom in full view of his fellow transgressors on the same palace wall which had been covered with cuneiform inscriptions glorifying those Babylonian kings. Compare Proverbs 16:18. His daring bravado was in an instant changed into abject fear; conscience can turn the most foolhardy into a coward. His promise that whosoever should read the writing should be "third ruler in the kingdom" is probably an undesigned coincidence with the historic truth now known that Nabonedus was the chief king, Belshazzar secondary, and so the ruler advanced to the next place would be THIRD (Daniel 5:7).
Daniel having been summoned at the suggestion of Nitocris, the queen mother, probably wife of Evil Merodach, Nebuchadnezzar's son, faithfully reproved him for that though knowing how God had humbled his forefather Nebuchadnezzar for God-despising, self-magnifying pride, he yet "lifted himself against the Lord of heaven"; therefore MENE, God has numbered thy years of reign and the number is complete, compare Psalm 90:12. TEKEL, weighed in the balances of God's truth, thou art found wanting. UPHARSIN, or PERES, alluding to the similar word "Persians," thy kingdom is divided among the Medes and Persians. Cyrus diverted the Euphrates into a channel, and guided by Gobryas and Gadatas, deserters, marched by the dry channel into Babylon, while the citizens were carousing at an annual feast to the idols (Isaiah 21:5; Isaiah 44:27; Jeremiah 50:29-35; Jeremiah 50:38-39; Jeremiah 51:36; Jeremiah 51:57). Belshazzar was slain; compare Isaiah 14:18-20.
Fausset, Andrew Robert M.A., D.D., "Definition for 'belshazzar' Fausset's Bible Dictionary".