Thinner than a thread of falsehood. أدق من خيط باطل

There are two interpretations: one is that it refers to dust particles that appear in sunlight and enter through a crack in the house; the other is that it is the thread that comes out of a spider's mouth, which children call "devil's sniffing." This second interpretation is better. Al-Jawhari said: "A thread of falsehood, the sun's saliva, and devil's sniffing are all the same." Marwan ibn al-Hakam was nicknamed "A Thread of Falsehood" because he was tall and unsteady, so he was given this nickname due to his thinness. A poet said of him: "May God curse a people who ruled over others like a thread of falsehood, giving to whomever he pleased and withholding from whomever he pleased." A tall person is also nicknamed "The Shadow of an Ostrich," just as he is nicknamed "A Thread of Falsehood."
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The proverb "Thinner than a spider's thread" is used to describe something extremely fine and delicate, whether referring to a spider's thread or the fine specks of dust seen in sunlight.
There is no exact English equivalent, but some of the closest expressions are:
As thin as a spider's thread.
As fine as a hair.
Hair-thin.
As delicate as a cobweb.
Thin as a cobweb.
The best equivalent that preserves the proverb's figurative meaning is:
As thin as a spider's thread.
If you want the closest English expression in meaning without being bound by the image, you can say:
Hair-thin.
Or:
As fine as a hair.
It indicates extreme precision and delicacy, as indicated by the Arabic proverb "more precise than a false thread".
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فيه قولان : أحدهما أنه الهباء يكون في ضوء الشمس فيدخل من الكوة في البيت والثاني أنه الخيط الذي يخرج من فم العنكبوت ويسميه الصبيان مخاط الشيطان وهذا القول أجود وقال الجوهري : خيط باطل ولعاب الشمس ومخاط الشيطان واحد وكان لقب مروان بن الحكم خيط باطل وذلك أنه كان طويلا مضطربا فلقب به لدقته وفيه يقول الشاعر :
لحا الله قوما ملكوا خيط باطل على الناس يعطي من يشاء ويمنع
والطويل أيضا يلقب بظل النعامة كما يلقب بخيط باطل

He went astray, son of a fool. ذَهَبَ فِي ضُلِّ بْنِ أُلٍّ‏

If he persisted in falsehood, it is said: He went astray, astray, and astray, if he went astray without right.
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This proverb is used to describe someone who has gone too far in their error and stubbornness, embarking on a path of misguidance without return. Some of the closest English equivalents are:
“He has gone off the deep end.”
Meaning: He has gone too far into error, madness, or extremism.
“To go down the wrong road.”
Meaning: To take the wrong path.
“He is deep in error.”
Meaning: He is deeply immersed in error and misguidance.
“To persist in folly.”
Meaning: He persists in foolishness and falsehood.
“He has lost his way completely.”
Meaning: He has gone completely astray.
The closest expression to the spirit of the Arabic proverb is:
“He has gone far astray.”
“He has gone too far into misguidance.”
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إذا ركبَ رأسَه في الباطل، يقال‏:‏ ذهب في الضَّلاَل والألال، والضلال والتلال، إذا ذهب في غير حق‏.‏