Judas

A traitorous action disguised as a show of affection. The phrase alludes to the Biblical account of the betrayal of Jesus by Judas, who kissed Jesus to identify him to the authorities arresting him. Don't try to be my friend now, it's just a Judas kiss! I know you're trying to get more information out of me so you can report it to the headmaster!

An action or relationship that is ultimately ruinous. For example, Some regard a royal divorce as a kiss of death to the monarchy. This term alludes to the betrayal of Jesus by Judas Iscariot, who kissed him as a way of identifying him to the soldiers who came to arrest him (Matthew 26: 47-49). It dates only from about 1940 but was previously called a Judas kiss.