What does Psalm 118:9 mean?

This statement is remarkably similar to the one just made (Psalm 118:8) but with a more specific focus. Only God is the perfect object of confidence and trust (Psalm 20:7; 56:11; 146:3). He is the ultimate reason for hope (Psalm 118:6; Hebrews 13:6). Human beings are unreliable, and many are deceptive and selfish (Romans 3:10). The need to trust God most of all applies even when we're tempted to trust powerful and influential people.

The word used here for "princes" is the Hebrew nedibim'. This means "rulers" or "officials." The lesson of this verse applies also to relationships between countries. Before the Hebrews entered Canaan, the Lord commanded them not to enter into any agreement with the inhabitants of Canaan (Exodus 23:32). Israel was to be a people separated unto the Lord with their trust placed firmly in Him. Second Kings 18 relates the story of how Hezekiah trusted the king of Assyria to spare the kingdom of Judah from aggression. Hezekiah assumed he could buy assurance that the king of Assyria would not attack Judah. It was misplaced confidence. After giving the king of Assyria all the silver that was in the temple and the royal treasury as well as the gold from the temple's doors and doorposts, the Assyrian king deployed a vast army to capture Jerusalem—a choice which did not end well for Assyria (2 Kings 19:35–36)