Psalm 35 is a plea for deliverance from enemies. Verse 21, "Yes, they opened their mouth wide against me. They said, “Aha! Aha! Our eye has seen it!”" illustrates the triumph and mockery of David's enemies over his suffering and perceived downfall. Here's a breakdown:

"Yes, they opened their mouth wide against me": This describes a posture of aggressive mockery, scorn, and boasting. Opening the mouth wide can suggest a loud, jeering laughter and unrestrained verbal attack. It emphasizes the intensity and open display of their animosity.

"“Aha! Aha!": This is an interjection expressing glee, satisfaction, and vindication at the misfortune of the speaker (David). The repetition intensifies the feeling. It's like saying, "We got you! We knew it!" or "We told you so!" They're reveling in his trouble.

"Our eye has seen it!": This suggests that they have witnessed David's suffering or humiliation firsthand. They have seen what they wanted to see—his defeat or disgrace. This is about evidence for them, evidence to prove their (likely malicious) claims or predictions about David. Their accusations, even if false, seemed validated by what they now observed. The word "eye" here makes it intensely personal and direct.

In essence, this verse depicts David's enemies exulting in his hardship, gloating over what they perceive as his deserved punishment or failure, and feeling vindicated in their negative opinions of him. The repetition and vivid imagery amplify the pain and humiliation David is experiencing.

Underlying Themes:

Innocence and Persecution: The verse implies that David is innocent of the charges against him and is being wrongly persecuted.
The Wicked's Rejoicing: It highlights how the wicked find joy in the suffering of the righteous.
The Need for Divine Intervention: It underscores David's need for God to intervene and deliver him from his enemies' malice.

So, in short, it's a picture of his enemies gloating and celebrating what they perceive as his failure, weakness, or deserved punishment.