Translate it: “Why is thy mighty one cast down? He stood not, because Yahweh thrust him down.” The “mighty one” is explained by the Septuagint to be the bull Apis. Thus:
(1) the chief deity of Egypt Jeremiah 46:15;
(2) the army of mercenaries Jeremiah 46:16;
(3) the king, Pharaoh Jeremiah 46:17, are the three upon whom the Egyptians trusted.