The phrase "kiss of death" can be used in a couple different ways. It can be used as an idiom, or figurative expression, that means an event or action that leads to ruin. It can also be used to describe a kiss an organized crime boss might give to another person, particularly when organized crime bosses are depicted in movies and in books. In such a case, a hard kiss is usually applied to the recipient's cheek or mouth as an indication that the boss is planning to kill the recipient or arrange for his death in the near future. In either case, it is easy to deduce that the meaning of the phrase is negative just from the inclusion of the word "death" in the phrase.
The most common use of the phrase "kiss of death" is as an idiom. In this case, a person uses the phrase to indicate that an event led to a negative result. Less often, this phrase is used to indicate a literal kiss. In such a case, the kiss described is applied to the cheek or mouth of a recipient and indicates serious trouble ahead. When applied by a person in organized crime, particularly someone in a position of authority, it usually means an attempt will be made to kill the recipient of the kiss.
As an example of the manner in which the phrase "kiss of death" can be used as an idiom, an individual could consider a politician who has just received the public support from an individual or group with a poor reputation. If the individual or group is held in poor enough regard by people most likely to vote for the politician, this endorsement might be referred to as "the kiss of death" to his chances of being elected. Likewise, the same effect could occur if voters learn of criminal history in a politician's past. In fact, even revealing a history of questionable behavior or decision making may kill a politician's chances of being elected.
An example of an organized-crime-related kiss of death could involve a criminal who was previously trusted as part of a band of criminals. If a person in authority learns that this criminal has betrayed the others in the group, he might deliver a kiss on the person's cheek or mouth, which indicates that he will likely suffer an attempt on his life. This sort of kiss is often portrayed in books and movies about organized crime.