The kiss of death, also referred to as a lipstick pistol, is a concealed firearm disguised as a lipstick container that was often used by the KGB.
EVA/Tatyana often carried this around, as well as an identical lipstick container in case she wanted to bluff someone into spilling info. When GRU colonel Yevgeny Borisovitch Volgin first met Tatyana nearing the end of the Virtuous Mission, Volgin, noticing that she was holding something in her hand, forced her to confiscate it, and discovered that she had a Kiss of Death. Because of its usage within the KGB, its discovery resulted in Major Ocelot immediately accusing Tatyana of being a KGB agent (as the KGB and the GRU were not on friendly terms at the time), although Volgin gave it back to her, as he saw some use with her. Later, she used her identical lipstick container to bluff Dr. Nikolai Sokolov into revealing where the Philosophers' Legacy was (or, at the very least, who knew where the Legacy was). Tatyana, later revealed as EVA, later attempted to use it on Colonel Volgin upon her identity being compromised, only for him to snatch it away.
Behind the scenes
The kiss of death was based on a real life spy weapon, of which at least one was confiscated at West Berlin.
Hideo Kojima planned to use several other real life spy gadgets from the 1960s during the development of Metal Gear Solid 3: Snake Eater. However, the development staff ended up dropping all except for the kiss of death from the final version, as they were forced to abandon going to the International Spy Museum due to the Beltway Sniper Attacks.
According to the commentary for the game, it was originally intended that Volgin use his eletricity to destroy the kiss of death when EVA tried to use it upon being exposed as a spy, as well as misfiring and having the bullet lodged into the ceiling of Groznyj Grad's hangar. In the actual game, however, he merely confiscates it.