own person, be one's
Also, be one's own man or woman. Be independent, be responsible for oneself. For example, We can't tell Jerry what to do—he's his own person. Chaucer used this idiom in Troilus and Cressida: “I am my own woman, well at ease.” [Late 1300s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.