Swim shops often offer a "grab bag" swim suit where you give them your size and they give you any one of their available suits. You do not get to choose which particular suit style or color you want. They are typically going to be older styles and non-popular colors, but they are going to be sold at a discount.
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Literally, a bag filled with unidentified things. If you reach into the bag, you might come out with anything -- a chocolate bar or a pocket comb, perhaps.
Figuratively, it's a situation where you have no idea what the consequence of your choice (which road to take, for example) might be.