All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The word curse means to be expelled from the mercy of Allaah, but it could also mean to insult. The Prophet said: "One of the great major sins is to curse your parents." The companions said: "O Prophet of Allaah, how can a person curse his parents?" The Prophet said: "It is when a person insults the father of another person who in return would insult his father, and insults his mother and in return he would insult his mother." [Al-Bukhari] Some scholars stated that if a person who commits a specific sin is cursed (in the Qur'aan and/or Sunnah), this means that this sin is one of the greatest major sins. Indeed they have defined a great sin with the following definition: "It is any sin that has a punishment in this world or a threat of being cursed or punished in the Hereafter or the like." Finally, it should be noted that it is not permissible to curse a specific Muslim even if he commits a sin that necessitates the curse, but it is permissible to curse a non-Muslim if he dies on disbelief (as a non-Muslim).
Allaah knows best.