Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is not true that biting nails brings misery or bad luck. Muslims are warned against believing in bad omen.

Imam Al Bukhari and Muslim reported that Abu Hurairah, may Allah be pleased with him, narrated that the prophet (Sallallahu Alahi Wa Sallam) said: "(There is) no 'Adwa (no contagious disease is conveyed without Allah's permission), nor is there any bad omen (from birds), nor is there any Hamah, nor is there any bad omen in the month of Safar."

Imam Al Nawawi, (Rahimahu Allah), said in his interpretation of the Hadith of Muslim, that seeing a bad omen means becoming pessimistic about something.

Initially, it is the thing people abhor. It can be a word, an action or a thing that can be seen. People used to associate bad omen with antelopes or birds. They used to chose them away and when these animals or birds go right they considered that as good omen and as result proceed in what they intended to do. On the other hand when these birds or animals went left, people considered that a sign of bad omen. Consequently, they refrained from traveling or fulfilling their needs and became pessimistic. So this prevented them most of the time from fulfilling their needs. So the Shariah (Islamic jurisprudence) forbade this and informed us that this does not have any effect whatsoever, it does not harm or bring benefit. This is the meaning of the Hadith of the prophet (Sallalahu Alahi Wa sallam), mentioned above.

He (Sallalahu Alahi Wa Sallam) also said in another Hadith:" To believe in bad omen is a form of Shirk". This means believing that it benefits or harms, and if one acts according to this belief thinking that it has some effect then this is Shirk as such a person considers it as having an effect on the act or causing an act to happen.

But, the Muslim should not cut his nails with his teeth, as there could be some dirt or germs in them (nails), and it is an unpleasant behavior.

Allah knows best.