וַיַּכְשִׁיל֣וּהוּ עָלֵ֣ימוֹ לְשׁוֹנָ֑ם יִ֝תְנֹדֲד֗וּ כָּל־רֹ֥אֵה בָֽם׃
So they make their own tongue a stumbling unto themselves; all that see them shake the head.
Rashi on Psalms
That which they hoped would make him stumble, their tongue brought upon them The stumbling they planned to inflict on him, their tongue turned over upon them.
Rashi on Psalms
will shake their heads Heb. יתנודדו, will shake their head; all who see them should nod their head and laugh about them.