With success assured; all over but the shouting.
The saying “in the bag” is new; that is, it has become generally known throughout America, where it originated, since about 1920.
In card games or the like, the person doing the playing may use it when he wishes to free his partner’s mind from anxiety. An earlier expression with the same inference was “all wrapped up”; that is, in allusion to merchandise, success was assured and merely awaited delivery.
When paper bags succeeded wrapping paper for the holding of groceries or the like, the later saying succeeded the older.