Armageddon is a real place. The word points to Megiddo, a hilltop city in northern Israel that armies fought over for thousands of years.
The name appears once in the Bible, in Revelation 16:16. There the kings of the world are gathered for "the war of the great day of God, the Almighty," at a place called Armageddon. The Greek text spells it Harmagedōn. It is read as the Hebrew Har Megiddo, "mount of Megiddo."
Megiddo had earned the association. Its ruins form a tell, a mound built up from 20 layers of settlement. The city guarded a pass on the most important trade route of the ancient Fertile Crescent, linking Egypt with Mesopotamia and Anatolia. Any army using that road had to deal with Megiddo.
The battles there are among the oldest on record. Around 1457 BCE, Pharaoh Thutmose III routed a Canaanite coalition led by the king of Kadesh, with the king of Megiddo among his allies. The UK National Archives calls it the first battle properly recorded. In 609 BCE, Egyptian forces under Necho II took Megiddo, and King Josiah of Judah fell in the fighting.
The name also holds a puzzle. The scholar R. J. Rushdoony pointed out that there are no mountains of Megiddo, only the Plains of Megiddo. He read that as a sign the vision was not meant literally. Others, including C. C. Torrey and James B. Jordan, trace it to Hebrew moed, "assembly," and link it to Sinai or Zion instead. Wikipedia sets out these readings, and none has settled the question.
English turned the place into a common noun. Etymonline dates the figurative sense, "cataclysmic final conflict," to 1811. The hill of Megiddo became a byword for the end of everything.
The old battlefield saw one more famous campaign. In September 1918, British and Indian troops under General Edmund Allenby broke through Ottoman defences in what became known as the Battle of Megiddo. The National Archives notes that there was not much more than a skirmish at Tel Megiddo itself. Allenby liked the name anyway. When he was made a viscount, he became Viscount Allenby of Megiddo.