Said to be so called because Judas Iscariot hanged himself thereon. This is one of those
word-resemblances so delusive to etymologists. Judæ is the Spanish judia (a French bean), and Arbor Judæ is a corruption of Arbol Judia (the bean-tree), so called from its bean-like pods.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894