Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† mŏnŏpōlĭum, ii, n., = μονοπώλιον, the exclusive privilege of dealing in or selling a thing, a monopoly (post-Aug.), Suet. Tib. 71; 30; Plin. 8, 37, 56, § 135.