(Dedicated to Marek)
As you know, rock is hard so being between a rock and a hard place is not a good place to be!
This American idiom describes being in a difficult situation with two unattractive choices.
As an Englishman, the rock that I think of is in fact a set of rocks on the Isle of Wight, off the south coast of England. It's famous for it's lighthouse.
Incredibly, the Navy used used to try to deliver a Christmas tree each year to the people stationed on the lighthouse.
Unfortunately, the crew found themselves between a rock and a hard place!