Strong's #4407: millow' (pronounced mil-lo')
 or mil-loe (2 Kings 12:20) {mil-lo'}; from 4390; a rampart (as filled in), i.e. the citadel:--Millo. See also 1037.
Brown-Driver-Briggs Hebrew Lexicon:
 ּ / ּ 
  millô' 
 
 Millo = "rampart" or "mound" 
 1) a place near Shechem; site unknown
 2) a part of the fortifications of Jerusalem
 
  Part of Speech: noun proper masculine
Relation: from H4390
Usage:
This word is used 9 times:
Judges 9:20: "the men of Shechem, and the house  of Millo; and let fire come out from the men"
Judges 9:20: "come out from the men of Shechem, and from the house  of Millo, and devour"
2 Samuel 5:9: "built round about from  Millo and inward."
1 Kings 9:15: "the house of the LORD, and his own house,  and Millo, and the wall of Jerusalem, and Hazor, and Megiddo,"
1 Kings 9:24: "Solomon had built for her: then did he build  Millo."
1 Kings 11:27: "Solomon built  Millo, and repaired the breaches of the city"
2 Kings 12:20: "Joash in the house  of Millo, which goeth down to Silla."
1 Chronicles 11:8: "the city round about, even from  Millo round about: and Joab repaired"
2 Chronicles 32:5: "without, and repaired  Millo in the city of David, and made darts"