’Where is he?’ said Frodo, looking round, as if he expected a masked and sinister figure to come out of a cupboard.
That cheered them greatly; for it was good to see a sight of anything beyond the wood’s borders, though they did not mean to go that way, if they could help it: the Barrow-downs had as sinister a reputation in hobbit-legend as the Forest itself.
There are no more uses of "sinister" in the book.
Show samples from other sources
They created a sinister plan to recruit terrorists.
In the movie, there was a sinister plot to kill people and harvest their organs.