a piece of a fighting man's gear, usually taken to be a shield: costly enough to be plated with gold, kept in royal and temple stores, and hung up on walls for display
The word turns up only seven times, always in the plural, and always among expensive military hardware. David strips the gold ones off the servants of a defeated king and carries them back to Jerusalem. Generations later a priest opens the temple stores and hands out spears and these same objects to the guards who will crown the boy king Joash. In Tyre, hired soldiers hang them along the city wall, and the prophet says the display is part of what makes the city beautiful.
That pattern tells us a great deal about the objects. They were portable, valuable enough to be gold plated and stored with treasure, numerous enough to hang in rows, and tied closely to a king's own troops. What the pattern does not tell us is their shape. Hebrew already had an ordinary word for a shield (מָגֵן) and a word for the tall body shield (צִנָּה), and 2 Chronicles 23:9 lists these objects right after the ordinary shields, as though they were something else again.
One verse pulls hard against the traditional picture. In Jeremiah 51:11 the summons is to sharpen the arrows and to fill these objects. You do not fill a shield. That single verb is the main reason a number of scholars read the word as a quiver or a bow case, and why some modern Bibles print "fill the quivers" there while others print "take up the shields." Both readings are alive, and the honest position is that the object behind the word has never been identified for certain.