THE COACH AND THE FLY. FABLE BY JEAN DE LA FONTAINE.
Up a long dusty hill, deep sunk in sand,
Six sturdy horses drew a Coach. The band
Of passengers were pushing hard behind:
Women, old men, and monks, all of one mind.
Weary and spent they were, and faint with heat;
Straight on their heads the sunbeams fiercely beat.
In the hot air, just then, came buzzing by,
Thinking to rouse the team, a paltry Fly.
Stings one, and then another; views the scene:
Believing that this ponderous machine
Is by his efforts moved, the pole bestrides;
And now upon the coachman′s nose he rides.
Soon as the wheels begin again to grind
The upward road, and folks to push behind,
He claims the glory; bustles here and there,
Fussy and fast, with all the toil and care
With which a general hurries up his men,
To charge the broken enemy again,
And victory secure. The Fly, perplexed
With all the work, confessed that she was vexed
No one was helping, in that time of need.
The monk his foolish breviary would read:
He chose a pretty time! a woman sang:
Let her and all her foolish songs go hang!
Dame Fly went buzzing restless in their ears,
And with such mockery their journey cheers.
After much toil, the Coach moves on at last:
"Now let us breathe; the worst of it is past,"
The Fly exclaimed; "it is quite smooth, you know;
Come, my good nags, now pay me what you owe."

So, certain people give themselves great airs,
And meddlers mix themselves with one′s affairs;
Try to be useful, worry more and more,
Until, at last, you show the fools the door.
https://e-libr.com/la-fontaine-the-coach-and-the-fly/

It describes someone who's agitated, enthusiastic and eager but without any real purpose, perhaps useless with a know-it-all attitude.
The reference explains that it's about being more of a hindrance or an obstacle instead of a helper.

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Note added at 1 day 20 hrs (2023-04-26 17:14:40 GMT)
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https://www.linguee.com/french-english/translation/la mouche...


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Note added at 1 day 21 hrs (2023-04-26 17:19:38 GMT)
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https://www.expressio.fr/expressions/la-mouche-du-coche

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Note added at 1 day 21 hrs (2023-04-26 17:22:49 GMT)
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In a traditional French fable the driver was of a coach drawn by six horses. One coach was for poor people only, who he transported most of the time, while the rich ones had their own special carriage.