When you use the expression ‘In the Bag’ you mean that something has been secured to
the point where it is almost in your possession.
Example of use: “Did you get the job you applied for?” Answer: “Not yet, but it’s in the bag.”
The origin of the phrase ‘in the bag’ comes from early 20th century America, when the New York Giants baseball team would carry a ball bag off the field when the team was in the lead, superstitiously thinking that the game was ‘in the bag’ and couldn't be lost.