What constitutes obscenity in speech, and is Imam Al-Suyuti considered obscene in his language for mentioning the names of private parts and details of sexual intercourse in his books, knowing that the Prophet, peace and blessings be upon him, forbade obscenity?

"Fuhsh" refers to that which is ugly in speech and action. Ibn Hajar defines it as exceeding the limit in bad talk. This includes explicitly mentioning matters of 'awrat (private parts) and what transpires between a man and his wife, which are normally considered shameful. As for the book "Nawadir al-Ayk fi Ma'rifat al-Nayk" attributed to al-Suyuti, there is doubt regarding its attribution to him because he did not list it among his works. Even if its attribution to him were correct, he might have written it early in his life and then recanted it, just as al-Suyuti himself mentioned that he washed away and retracted some of his works. The infallible is the divine revelation, and everyone's sayings can be accepted or rejected except for the Prophet Muhammad, peace and blessings be upon him.