unless someone can tell me that there are feminine words out there with an -us ending.
The fourth declension is a group of nouns consisting of mostly masculine words such as fluctus, fluctūs m. ('wave') and portus, portūs m. ('port') with a few feminine exceptions, including manus, manūs f. ('hand').
https://en.wikipedia.org/wiki/Latin_declension#Fourth_declension_(u_stems)