Literally, it means you have turned over and looked beneath every possible stone. Idiomatically, it means you've looked everywhere you could and have done everything you could to try to solve some problem.
i dont know! from zamzam year 8
1 stone = 14 poundsstone
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It is a term used in Stonemasonry. It refers to a stone cross used to mark a grave. A blank Stone Cross, simply means it has no inscription on it.