upas is a (fabulous?) tropical tree which yields a poisonous latex (used on arrows) that goes by the same name. M-W gives upas a second meaning, a poisonous or harmful influence or institution.

T. E. Lawrence seems to use it as an adjective [or modifying noun, if you will] in the latter sense, in Seven Pillars..:

We had been hopelessly labouring to plough waste lands; to make nationality grow in a place full of the certainty of God, that upas certainty which forbade all hope.