upas Syllabification: (u·pas)Pronunciation: /ˈyo͞opəs/ (also upas tree)Definition of upas noun a tropical Asian tree, the milky sap of which has been used as arrow poison and for ritual purposes. Antiaris toxicaria, family Moraceae (in folklore) a Javanese tree alleged to poison its surroundings and said to be fatal to approach. Origin: late 18th century: from Malay (pohun) upas 'poison (tree)'upas in other Oxford dictionaries Definition of upas in the British & World English dictionary