John Gill Commentary Psalms 35:21

"Yea, they opened their mouth wide against me; They said, Aha, aha, our eye hath seen it." — Psalms 35:21 (ASV)

Yea, they opened their mouth wide against me
In laughter, scorn, and derision; see (Psalms 22:7Psalms 22:8) ;

[and] said, Aha, aha :
a word expressive of joy; and the doubling it shows the greatness of it;

our eye has seen [it] ;
what their heart wished for; namely, the distress of him, whose enemies they were.