"insanity in regard to a single subject or class of subjects; mental action perverted to a specific delusion or an impulse to do a particular thing," 1820, probably on model of earlier French monomanie (1819), from Modern Latin monomania, from Greek monos "single, alone" (from PIE root *men- (4) "small, isolated") + mania (see mania).
Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]
<a href="https://www.etymonline.com/word/monomaniac">Etymology of monomaniac by etymonline</a>
Harper, D. (n.d.). Etymology of monomaniac. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/monomaniac