개똥도 약에 쓰려면 없다
dog-shit-TOO medicine-AS use-intend-when there-isn't
Even when you want to use dogshit as medicine, there isn't any.
This is basically a proverb that means that no matter how minor your need might be, or how much you are willing to yield in the quality of what's needed, in the moment of need, the needed thing is missing. It's kind of like a version of Murphy's Law: "If it can go wrong, it will."