Originally the meaning was to open a keg of "whiskey" for a party. It means "Let's have some fun"

I heard open a keg of nails. It mean's to get the party started. E.g: My Dad used to tell us kids that If you all behave we'll stop up the road and open a keg of nails. Hope this helps :)

It`s simpler than you think. The 'keg' is also used with the weight of nails and equals to 100 lb (pounds) of nails. It means just: "Buy 100lb of nails".

I've never heard that expression either :)