Douglas Harper's Etymology Dictionary
sui juris
1610s, "of full legal age and capacity," in ancient Rome, "of the status of one not subject to the patria potestas." For first element, see sui generis; for second element, see jurist.
Wiktionary
sui juris
a. Having the right and the capability to manage one's own affairs
Usage examples of "sui juris".
Pray earnestly to God to prosper him in his Voyage, and to keep him from danger, and, if he be 'sui juris' he should make his last will, and wisely order all his affairs, since many that go far abroad, return not home.