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.
You argued that man DOWN! You were in your BAG! He was not ready.
An insult originally coined in the mid 1700's by Anglo-Irish writer Jonathan Swift. The insult has no real defined meaning but It can be determined from context to essentially be accusing ones mother of being a possession. As was commonplace then and now many people carry their possessions in bags, thus the insult begins to make sense.
While not commonly used, the insult has begun to make an appearance on many social media sites such as facebook, instagram, imgur, youtube etc.
"I know your mother well. She's in a bag" or "Your mum in a bag"
To surripiteously smoke a bowl on the beach by getting low to the ground and sticking your head in your backpack like you are looking for something. You then proceed to light up.