The shortened way of saying 'for fuck's sake'. Just saying sakes is a very simple way to get your point across when something is wrong or if you are angry. Sakes
{John}: Hey Josh, quick! Check out that blimp about to crash into a radio tower!
{Josh}: For Christ sakes John, that looks just like the pictures of the Hindenburg crash that I've seen!!!
Used as a general term of exclamation or surprise similar to oh my lord, or oh my goodness, or oh shit. Mostly used by people in the 1930's, 1940's, and 1950's. Possibly a minced oath (softened form of swearing) substitute for "For the Lord's sake."