This box contained an unctuous substance partly solid, of which it was impossible to discover the color, owing to the reflection of the polished gold, sapphires, rubies, emeralds, which ornamented the box.
There are no more uses of "unctuous" in the book.
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But she kept her attention on me, unctuously gesturing, and even as he leaned and bucked into her she held out her open hand to me, as if I should take it when I crept my way inside.
Chang-rae Lee -- A Gesture Life
He made no effort to be civil, or his normal unctuous, ingratiating self.