He barked wildly, his backside wet. دَهْوَرَ نَبْحاً واسْتُهُ مُبْتَلَّةٌ.

The barking of a dog: A dog that barks, farts, and defecates out of fear of a lion.
This proverb is used to describe someone who threatens someone stronger and more powerful than himself.

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The proverb "He barked loudly, but his backside was wet" is used to describe someone who displays threats, intimidation, and outward bravery, while in reality, he is fearful and trembling before someone stronger.
There is no exact English equivalent, but the closest English proverbs and expressions in meaning are:
His bark is worse than his bite.
— Said of someone who makes many threats but lacks the ability to carry them out.
All bark and no bite.
— This is the closest equivalent to the Arabic proverb.
Empty vessels make the most noise.
— Said of someone who talks a lot and makes threats without any real power.
Bravado is not bravery.
The best and most common English equivalent is:
All bark and no bite.
Because it portrays a person who threatens and makes promises, but in reality is a coward in the face of someone stronger than him, which is exactly what the Arabic proverb “He barks for ages while his backside is wet” means.
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الدهورة: نُبَاح الكلب من فَرَق الأسد ينبح ويَضْرُط ويَسْلَح خوفاً منه.
يضرب لمن يتوعَّدُ من هو أقوى منه وأمنع