|Easton's Bible Dictionary|
A highway; a twig, only in 2 Kings 12:20. If taken as a proper name (as in the LXX. and other versions), the locality is unknown.
Int. Standard Bible Encyclopedia
sil'-a (cilla'; Codex Vaticanus Galla; Codex Alexandrinus Gaallad): Joash was assassinated by his servants "at the house of Millo, on the way that goeth down to Silla" (2 Kings 12:20). Wherever Beth-millo stood, Silla was evidently in the valley below it; but nothing is known of what it was or where it stood.
Silla (1 Occurrence)
2 Kings 12:20 His servants arose, and made a conspiracy, and struck Joash at the house of Millo, on the way that goes down to Silla. (WEB KJV JPS ASV BBE DBY WBS YLT NAS RSV NIV)