To do something with confidence, sauce, skill, flair, and excellence.
Related to: “Getting your
bag” meaning getting your
money. “In your
bag” is related because it means you’re in your element doing something with the skill and swag of a pro. It means you’re in your element.
Slang history: Related to the 50s slang to have something “in the
bag”, which meant it already accomplished, or so assured with confidence, it was as good as done.