What is the aspect of truth mentioned in Imam Al-Nawawi's commentary, and is the aforementioned utterance originally present in the science of Jarh (discrediting narrators), and is it permissible to use the word "dog" to describe someone who made a mistake in issuing a fatwa or a group of fatwas?

It is not permissible to curse a Muslim without a legitimate Shar'i right, and doing so is an act of transgression (fisq). Allah does not love the utterance of evil speech except by one who has been wronged, and the recompense of an evil deed is an evil deed like it. As for al-Jarh wa at-Ta’dil (discrediting and accrediting narrators), it must be limited to what achieves the benefit, without exceeding to the faults of the narrator. This must be done for the purpose of sincere advice and seeking the pleasure of Allah, not out of personal whims. It is not permissible to use vulgar language against someone who errs in their fatwa; rather, it is obligatory to advise and guide them. If they do not accept, then their fatwas should be warned against, and the discrediting (jarh) should be limited to the point of error. Whoever curses a Muslim without right is a transgressor (fasiq).