E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.