Judas

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who betrays another under the guise of friendship.
• noun A one-way peephole in a door.

from The Century Dictionary.

• noun A treacherous person; one who betrays under the semblance of friendship.
• noun [lowercase] In some old houses, a lattice with small openings in a door, through which those inside could look without, being seen: designed to prevent the admission of objectionable persons.
• noun Hence In a prison, a small opening in the door or wall of a cell to enable the guards to watch the prisoners; a judas-hole.

from the GNU version of the Collaborative International Dictionary of English.

• noun The disciple who betrayed Christ.
• noun a peephole or secret opening for spying.
• noun an act appearing to be an act of friendship, which is in fact harmful to the recipient.
• noun (Bot.) a leguminous tree of the genus Cercis, with pretty, rose-colored flowers in clusters along the branches. Judas is said to have hanged himself on a tree of this genus (Cercis Siliquastrum). Cercis Canadensis and Cercis occidentalis are the American species, and are called also redbud.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun biblical One of the twelve original Apostles of Jesus, known for his role in Jesus' betrayal into the hands of Roman authorities. Also called Judas Iscariot.
• proper noun A male given name.
• noun A traitor, a person not to be trusted.
• noun A small hole in a door through which a person can look without being seen from the other side; used especially in prisons; a judas-hole

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who betrays under the guise of friendship
• noun (New Testament) the Apostle who betrayed Jesus to his enemies for 30 pieces of silver
• noun a one-way peephole in a door
• noun (New Testament) supposed brother of St. James; one of the Apostles who is invoked in prayer when a situation seems hopeless