usus

Usus, usus, m., fourth declension: genitive usus, dative usui, ablative usu, and the nominative and genitive singular look identical, which trips people up. It is a fourth-declension verbal noun built on the supine stem of utor, us-, the same stem the participle usus comes from, so it means the using of a thing, the practice built up by using it, and the experience that practice leaves behind.

Two constructions repay learning. Usui esse alicui is a double dative, "to be of use to someone", and it takes esse plus two datives with no accusative anywhere. And usus est with the ablative means "there is need of", the same pattern as opus est. In Roman law usus is possession established simply by using something over time, which is where usucapio and usufruct come from, and English use, usage, utility and usual all sit here.