🔼The name Millo in the Bible
There are one or two Millos in the Bible, not counting the Beth-millo mentioned in Judges 9:6. And it's not even sure that Millo is an actual name. It's consistently preceded by the definite article — המלוא, meaning the Millo — which suggests that millo is not a name but a regular word. The article is used in Hebrew to distinguish one thing from the thing we're talking about, but the Millo we'll discuss below appears to have been unique. Another reason why the article precedes this word may be to indicate that the millo was executing the action represented by the verb the word millo came from: the thing that does millo.
The first Millo we hear of appears to be either a man-made construction or a natural rock formation on the slopes of Mount Zion:
When David captured the stronghold and associated village of Zion from the Jebusites and moved in, he expanded into the city of Jerusalem, building from the millo onward (2 Samuel 5:9, 1 Chronicles 11:8). But when Solomon began his building project that included the temple and the royal palaces, he was also said to have built the millo (1 Kings 9:15, 9:24). Perhaps he fortified the millo that David built, perhaps he built a whole other millo, and perhaps the millo of Solomon was applied back to the story of David as a reference.
Still, Solomon's construction work pertaining to the millo is of crucial importance in the story of Israel. In 1 Kings 11:27 we learn that Jeroboam, son of Nebat, began the rebellion that resulted in the breach between Judah and the northern kingdom of Israel because Solomon was building the millo.
King Joash of Judah was killed in or near his house in millo (בית מלא, no definite article; 2 Kings 12:20, see 2 Chronicles 24:25), and after the Assyrian invasion, king Hezekiah bravely repaired the wall of Jerusalem and the millo in the city of David (2 Chronicles 32:5).
Why the construction work on the millo was so vexing to Jeroboam that he tore the united kingdom apart, and subsequently ended Israel's golden age may not be immediately clear. Jeroboam was chief of the forced labor coming from Ephraim and Manasseh (the house of Joseph; 1 Kings 11:28, but see 11:22), and although Jeroboam was vexed, it was YHWH Himself who gave him the northern kingdom (1 Kings 11:31). It therefore appears that not solely Solomon's many wives and their idols led to the undoing of the united kingdom but also Solomon's work on the mysterious millo.
🔼Etymology of the name Millo
The name Millo appears to be derived from the verb מלא (male'), meaning to be full or be filled:
For a meaning of the name Millo, NOBSE Study Bible Name List reads Terrace, Elevation and Jones' Dictionary of Old Testament Proper Names has Rampart, but note that these are all interpretations an not translations. BDB Theological Dictionary doesn't offer an interpretation of our name but does list it under the verb מלא (male'), meaning to be full or be filled
The form of the word millo looks like an absolute infinitive or a passive participle, and it means Filled, Filling, or The Setting Up. The form מלוא appears six times in the narrative of the Bible: Judges 6:38, 1 Chronicles 16:32, Psalm 24:1, Jeremiah 8:16 and 47:2, consistently meaning 'fullness' or 'that contained'.
The Millo was not a landfill (because why would a landfill have a name; it's just a foundation), but a storage facility. And since it's preceded by the definite article and perhaps also contained lodging quarters, it was a heavily fortified general storage complex; the storage complex of the city. It probably contained food, and perhaps even water, and possibly armor and a "panic room," which would explain why Joash went there.
And it also explains Jeroboam's anger. From the account of Solomon's life it's obvious that the revered peace-king was in fact a relentless megalomaniac. He gathered more wealth (and women) than anyone would ever need, but those treasures and women needed housing. Jeroboam saw the millo expand and expand without any good reason: "So Solomon built [...] all the storage cities which Solomon had, even the cities for his chariots and the cities for his horsemen, and all that it pleased Solomon to build in Jerusalem, in Lebanon, and all the land under his rule" (1 Kings 9:19).
Solomon's empire enjoyed an annual revenue of 4,000,000,000 USD in today's money (10:14), and Solomon amassed 1,400 chariots and 12,000 horsemen, which he stored in dedicated cities and with him in Jerusalem (10:26). He had acquired 700 wives and 300 concubines (11:3), which needed an untold number of servants and produced an untold number of offspring. And they required their own temples, which Solomon gladly provided for. The Lord became angry because Solomon accommodated and honored the idols of his wives, but Jeroboam became angry because he and his men had to construct all this foolishness. Their suffering must have been horrendous.
When Solomon died, Jeroboam addressed the new king Rehoboam, and said, "Your father made our yoke hard; now therefore lighten the hard service of your father and his heavy yoke which he put on us, and we will serve you" (12:4).
Rehoboam's response was as ridiculous as his celebrated "wise" father's megalomania: "I will add to your yoke! My father disciplined you with whips; I will discipline you with scorpions" (12:11).
The kingdom was lost because the proverbially wise king used his wisdom to write pretty poetry for the aristocracy and adorn wealthy women. And the poor slobs on the work floor were footing the bill. To them Israel had never escaped from Egypt; they had merely swapped one arrogant king for the next.
Much later, another King would arise, who would become compared with king Solomon. But because He knew how men lose empires, he said, "Do not lay up for yourselves treasures upon earth" (Matthew 6:19), and "Come to Me, all who are heavy-laden, and I will give you rest. Take My yoke upon you and learn from Me, for I am gentle and humble at heart, and you shall find rest for your souls. For My yoke is easy, and My load is light" (Matthew 11:28-30).
Everybody in Jesus' original audience would have realized that He was referring to Solomon and his infernal millo.