Similar to the idea behind "don't cry over spilled milk", this phrase is widely useful. In a situation where something sucks for person A, person B might have the feeling like, well what are you gonna do, you cant do anything, so get over it, as in shit in a bag, man, there's nothing you can do about it. Thus, person B would say, shit in bags.
Props to Jen Smith, of Chicago, IL.
Person A: "My bike just got stolen!"
Person B: "shit in bags, man, nothing you can do about it."
When somebody dumps an unpleasant task on you with the expectation that you must immediately do something about it, especially when the task in question is completely outside your ordinary area of responsibility.
My elderly father lives 150 miles away but he just called telling me he has fallen on the floor and can't get up, as usual he gives me more bags of shit to hold.
When someone is being very Cryptic and withholding important information or news.
This person knows that you desperately need this information, but as they are a Cryptic Shitbag they release only tiny bits of information, despite many hours of questioning, politeness and bribery.
They may imply and insuate possible answers, but often these answers only contain the smallest nuggets of truth.
These people are Cryptic Shitbags.
Rufus: You were gone a long time, did your Mum tell you when and where the Funeral will be?
GF: No. She's playing Cryptic Shitbags.