A wolf alone is a lion. الذِّئْبُ خَالياً أَسَدٌ‏.‏

It is also narrated as "more aggressive," meaning that if it finds you alone, it will be bolder towards you. This is a saying attributed to some. A better interpretation is that a wolf, when alone and without helpers of its own kind, is like a lion, because it relies on its inherent strength and power, leaping with a ferocity that leaves no trace. This is closer to the truth, because "alone" describes the state of the wolf, not something else. The intended meaning is: a wolf resembles a lion when alone, just as you might say, "Zayd is like the moon when laughing." The comparison here governs the state. Abu Ubayd said: He means that if it has power over you in this situation, it is stronger and bolder in its oppression. That is, in other circumstances, it means you are not incapable of attacking it, nor does it have any helper of its own kind. He also said: This proverb is sometimes used In religion, and from it is the hadith of Mu'adh, may God be pleased with him: "Adhere to the community, for the wolf only preys on the stray sheep that wanders off." Abu Ubaid said: This proverb became applicable to matters of religion and worldly affairs. It is used to describe anyone who isolates himself in his opinion, his religion, or his travels.
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This proverb, "A lone wolf is a lion," especially with the explanation provided, is used to describe someone who isolates themselves from the group and becomes vulnerable, or someone who relies solely on themselves without support. It is also used to warn against isolation, acting alone, or traveling.
Some of the closest English equivalents are:
"A lone sheep is easy prey for the wolf."
Meaning: A lone sheep is easy prey for the wolf.
This is very similar to the aforementioned hadith of Mu'adh.
"The wolf catches the stray sheep."
Meaning: The wolf hunts the stray sheep.
"There is safety in numbers."
Meaning: Safety lies in unity.
This is a well-known English proverb that corresponds to the general meaning.
"United we stand, divided we fall."
Meaning: United we stand, divided we fall.
This also conveys the meaning of warning against division.
"No man is an island."
Meaning: No one can live entirely on their own.
The closest equivalent to the proverb in the context of warning against isolation is:
“There is safety in numbers.”
“Safety lies in the group.”
However, if the intended meaning is:
“The wolf catches the stray sheep.”
“The wolf preys on the stray sheep.”
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ويروى ‏"‏أشَدُّ‏"‏ أي إذا وجَدَك خاليا وَحْدك كان أَجْرَأ عليك، هذا قول قاله بعضهم‏.‏
وأجود من هذا أن يقال‏:‏ الذئب إذا خلا من أَعْوانٍ من جنسه كان أسداً، لأنه يتكل على ما في نفسه وطبعه من الصَّرَامة والقوة فَيَثِب وَثْبة لا بُقْيَا معها، وهذا أقرب إلى الصواب، لأن ‏"‏خالياً‏"‏ حال من الذئب لا من غيره، والتقدير‏:‏ الذئب يشبه الأسد إذا كان خالياً، كما تقول‏:‏ زيد ضاحكا قمر، ومعنى التشبيه عامل في الحال، قال أبو عبيد‏:‏ يقول‏:‏ إذا قَدَرَ عليك في هذه الحال فهو أقوى عليك وأجرأ بالظلم، أي في غير هذه الحال، أراد لا تَعْجِزْ عنه ولا معين له من جنسه‏.‏
وقال أيضاً‏:‏ قد يضرب هذا المثل في الدِّينِ، ومنه حديث معاذ رضى اللّه تعالى عنه ‏"‏عليكم بالجماعة فإن الذئب إنما يُصيب من الغنم الشَّاذَّةَ القاصِيَة‏"‏ قال أبو عبيد‏:‏ فصار هذا المثل في أمر الدين والدنيا‏.‏
يضرب لكل متوحِّدٍ برأيه أو بِدِينه أو بسفره‏.