Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mūgĭōnĭa or Mūgōnĭa, ae, or Mūgĭōnis (Mucionis), is, f. (porta), a gate of ancient Rome, which was afterwards within the city: in Palatio Mucionis a mugitu, Varr. L. L. 5, § 164 Müll.: Mugionia porta Romae dicta est a Mugio quodam. Paul. ex Fest. p. 144 Müll.; Sol. 1, 24; cf. Becker’s Antiq. 1, p. 109 sq.