I'm just read The Hound of the Baskervilles, and in few sentences it's said about "in thunder" or "by thunder".
"Now," said Sir Henry Baskerville, "perhaps you will tell me, Mr. Holmes, what in thunder is the meaning of that, and who it is that takes so much interest in my affairs?"
and also this
"And yet, my dear Watson, there is so very close a connection that the one is extracted out of the other. 'You,' 'your,' 'your,' 'life,' 'reason,' 'value,' 'keep away,' 'from the.' Don't you see now whence these words have been taken?"
"By thunder, you're right! Well, if that isn't smart!" cried Sir Henry.
So, what is that mean?