The Judas tree was a tree from which a deadly poison could be extracted. Trained by the Silence, River Song kissed the Eleventh Doctor with lipstick made from the poison of the Judas tree. (TV: Let's Kill Hitler)
Behind the scenes Edit
Judas was the name of a disciple in the Bible who betrayed Jesus Christ with a kiss. To call someone a "Judas" is to mark them as a traitor or backstabber. Fittingly, the Judas tree is used to poison and betray people, and in this case administered with a kiss.