Whack, out of

"Things are out of whack," said a friend.  I knew what he meant--the situation was disordered and messy.  And I knew what a whack was--it's a strong blow or strike.  But what a strange word--whack?  Where does it come from?  And why can something be out of whack but not IN whack?

The word can be traced to the 1700's, and comes into English from the Scottish.  That much I know.  But as to the rest--sorry, that's a mystery!