sure of oneself
Self-confident, as in Now that Mary's graduated she's much more sure of herself. This expression uses sure in the sense of “confident” or “secure,” a usage dating from the mid-1400s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.