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. The symptoms of the poison as it progressed through the Doctor's system appeared to be extremely painful. (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.