God strike him with the third of the three stones رماه الله بثالثة الأثافي

They said: It is a piece of rock placed next to two other stones, upon which the cooking pot rests. This is said of someone who inflicts a great calamity, and also of someone who leaves no evil untouched, because the three stones of the three stones are each the size of a person's head. So, if he strikes him with the third, he has reached the end. This is what al-Azhari said. Al-Badi' al-Hamadhani said: "I have a body like one of the three stones... and a liver like the third of the three stones."
He means the piece of rock.

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 The closest English equivalent to this proverb is:
The last straw.
Meaning: The final, devastating blow.
Another expression very close to conveying the meaning of a great calamity is:
A crushing blow.
Or:
Hit with the worst calamity.
Because the phrase "the third of the three stones" signifies the ultimate in severity and hardship, there is also:
When it rains, it pours.
Meaning: When misfortunes come, they come all at once.
It can also be expressed as:
The final and greatest disaster.
This is closer to the original meaning of the Arabic proverb.

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قالوا: هي القطعة من الجبل يوضع إلى جنبها حجران وينصب عليها القدر.يضرب لمن رمى بداهية عظيمة، ويضرب لمن لا يبقى من الشر شيئا، لأن الأثفية ثلاثة أحجار كل حجر مثل رأس الإنسان، فإذا رماه بالثالثة فقد بلغ النهاية، كذا قاله الأزهري، قال البديع الهمذاني:
ولي جسم كواحدة المثاني ... له كبد كثالثة الأثافي.
يريد القطعة من الجبل

It is a viper, a real snake.......إنه لصل أصلال

  A viper is one that kills instantly if it bites. It is used to describe someone cunning. The poet said:

What a calamity we have suffered from a male snake, a viper that kills instantly if it bites..

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Some of the closest English equivalents are:

A snake in the grass.

Meaning: "A snake in the grass." This is said of someone who is cunning and secretive.

As sly as a serpent.

Meaning: "As cunning as a snake."

A deadly operator.

Meaning: "A very dangerous and cunning person."

He's venomous.

This is used to emphasize malice and wickedness.

To indicate cunning mixed with danger:

A dangerous cunning man.

A more accurate translation of the proverb is:

He is a deadly and cunning man, like a venomous serpent.

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الصل: حية تقتل لساعتها إذا نهشت. يضرب، للداهي. قال الشاعر:

ماذا رزئنا به من حية ذكـر

نضناضة بالمنايا صل أصلا