a Judas kiss
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!
a Judas kissan act of betrayal, especially one disguised as a gesture of friendship.
Judas Iscariot was the disciple who betrayed Jesus to the authorities in return for thirty pieces of silver: ‘And he that betrayed him gave them a sign, saying, Whomsoever I shall kiss, that same is he: hold him fast’ (Matthew 26:48).