A person who is obsessed with a single thing, to the exclusion of other concerns.
To talk nonsense, or poetry, or the dash between the two, in a tone of profound sincerity, and to enunciate solemn discordances with received opinion so seriously as to convey the impression of a spiritual insight, is the peculiar gift by which monomaniacs, having first persuaded themselves, contrive to influence their neighbours, and through them to make conquest of a good half of the world, for good or for ill.1859, George Meredith, chapter 15, in The Ordeal of Richard Feverel. A History of Father and Son. […], volume (please specify |volume=I to III), London: Chapman and Hall, →OCLC:
Harry knew now that the old man was crazy, or at least a monomaniac, and, though he seemed harmless enough, it was of course possible that he might be dangerous.1873, Horatio Alger, chapter X, in Bound to Rise:
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He had never yet been able to determine whether the old man was a consummate hypocrite or a religious monomaniac.1890, Arthur Conan Doyle, chapter 44, in The Firm of Girdlestone:
monomaniac
adj
1
Focused on one thing above all others.
...then it was that monomaniac Ahab, furious with this tantalizing vicinity of his foe, which placed him all alive and helpless in the very jaws he hated; frenzied with all this, he seized the long bone with his naked hands, and wildly strove to wrench it from its gripe.1851 November 14, Herman Melville, chapter 133, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, →OCLC: