AND EVEN BETTER, AN ACTUAL OBSCENE SAYING:
Something you can swear by, used in a way similar to "by God!" It seems to have come from seafaring slang, and might refer to the Big Dipper. But you don't need to know the origin to find it useful. Today the strange randomness of the words makes it feel mystically satisfying to shout.
If you haven't read that book, btdubs, YOU ARE TOTALLY MISSING OUT. BECAUSE IT IS AWESOME.