"And the smell, A said, because he sniffs the cheap rose perfume on Gerty's piece of cotton wool, and thinks of the one Molly uses. Opoponax! B exclaimed. And at the opening of his waistcoat he also smells the lemon scent of his cake of soap."
The House of Ulysses by Julián Ríos, translated by Nick Caistor, p 181
Stephen King has tried to repurpose this word, c.f. Wikipedia:
* In the novel Black House by Stephen King and Peter Straub, the word opopanax is used repeatedly and constantly in a nonsensical fashion, as both a verb and an adjective (e.i "distant cry of the opoponax", the opoponax this, the opoponax that, etc) eventually becoming a symbol for all the strange and incomprehensible events unfolding in the book.
* In another Stephen King novel, Wolves of the Calla (the fifth book in The Dark Tower series), a character holds an "opopanax feather", thus suggesting that it is the name of a bird. No other explanation is given in the story.