The wolf is nicknamed Abu Ja'dah. الذَّئْبُ يُكْنَى أَبَا جَعْدَةَ‏.‏

It is said that Ja'dah is a young sheep, and the wolf is nicknamed after it because it targets and seeks out sheep due to their weakness and gentleness. It is also said that Ja'dah is a fragrant plant that grows in spring and dries quickly. Similarly, if a wolf is honored with a nickname, it will quickly become treacherous and will not remain in one state. Another interpretation is that the wolf, though its nickname may be good, acts reprehensibly. Yet another explanation is that it originated with 'Ubayd ibn al-Abras when Nu'man ibn al-Mundhir wanted to kill him. This proverb is used to describe someone who flatters with words but intends harm. Ibn al-Zubayr was asked about temporary marriage (mut'ah), and he said: "The wolf is nicknamed Abu Ja'dah," meaning that it is a good nickname for a cunning wolf. Likewise, temporary marriage has a good name but a bad meaning.
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The proverb "The wolf is called Abu Ja'dah" is used to describe someone who feigns kindness and pleasant speech, or who bears a good name, while concealing malice and deceit, or someone who speaks well but harbors ill intentions.
One of the closest English equivalents is:
"A wolf in sheep's clothing."
This is the most famous and closest English proverb to this meaning.
"Fair words, foul deeds."
"Honey in the mouth, poison in the heart."
"Beware of a smiling enemy."
"Fine feathers do not make fine birds."
The closest equivalent to the Arabic proverb is:
"A wolf in sheep's clothing."
"A wolf in sheep's clothing."
Because it combines outward goodness with inward malice, which is exactly what the Arabic proverb meant.
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يقال‏:‏ إن الجَعْدَة الرِّخْلُ، وهي الأنثى من أولاد الضأن، يكنى الذئب بها لأنُه يقصدها ويطلبها لضعفها وطيبها، وقيل‏:‏ الجَعْدة نبت طيب الرائحة ينبت في الربيع ويجفُّ سريعاً، فكذلك الذئب إن شَرُف بالكنية فإنه يغدر سريعاً، ولا يبقى على حالة واحدة، وقيل‏:‏ يعني أن الذئب وإن كانت كنيته حسنة فإن فعله قبيح، وقيل‏:‏ إنه لعَبيد بن الأبرص قاله حين أراد النعمان ابن المنذر قتله‏.‏
يضرب لمن يبرك باللسان ويريد بك الغَوَائل‏.‏
وسئل بن الزبير عن المتعة، فقال‏:‏ الذئب يكنى أبا جعدة، يعني أنها كنية ‏حسنة للذئب الخبيث، فكذلك المتعة حسنة الاسم قبيحة المعنى‏.‏