Finer than flour. أدق من الطحين

This is the active participle of the passive participle, meaning finely ground. The preceding phrase refers to fineness. This is from the words of the poet Al-Hutay'ah, addressing his mother:
You have held sway over your sons until... you have left them finer than flour.
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The Arabic proverb "fineer than flour" refers to extreme fragmentation, weakness, and utter annihilation, as in Al-Hutay'ah's verse, meaning: You humiliated and weakened them until they became utterly feeble.
The closest English equivalents, according to the context, are:
Ground to dust.
Crushed to dust. — This is the closest equivalent in terms of both imagery and meaning.
Crushed to powder.
Reduced to nothing.
Eliminated or weakened to the point of insignificance.
Beaten to a pulp.
Completely exhausted or crushed (if the intention is humiliation or defeat).
Therefore, the best equivalent for the proverb in the context of Al-Hutay'ah's verse is:
They were ground to dust.
Or:
You crushed them to powder.
Both convey the meaning of "You left them finer than flour," that is, weaker and more humiliated than imaginable.
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هذا أفعل من المفعول وهو المدقوق وما تقدم فمن الدقة وهذا من قول الشاعر الحطيئة يخاطب أمه :
وقد ملكت أمر بنيك حتى ... تركتهم أدق من الطحين