In the Middle Ages, people swore a lot. Not they way we think of swearing, although they did their share of cussing as well.
These days, it seems only thieves and druggies who are trying to convince the cops they are fine, upstanding citizens and start babbling, “I swear,” or when they are REALLY desperate, “I swear on my mother’s life!”
Back in dem olden days, God had a much bigger role in everyday life. And to convince people of their truthfulness, or their honesty, THEY would swear. To make the oath even more impressive, they’d tack on “By the Blood of God,” “By the Wounds of God.” Folks figured when you started talking like that, you HAD to be telling the truth, or God would send down lightning bolts and fry you on the spot.
People also discovered that when they DIDN’T get hit by lightning, they could spice up their profanity by putting similar phrases. “God’s wounds,” “God’s blood,” and the ever-creative “God’s toenails.”
Sometimes you’ll hear an old-fashioned expletive, “Zounds!” That is actually a contraction of “God’s wounds.” British people like to get all heated up and start talking about “bloody this,” and “bloody that.” Americans think those phrases are rather hilarious, if not messy. But the British “bloody” is derived from the oath-making “God’s blood,” and using it casually is borderline blasphemy.
“Sacre bleu” falls into the same category. It’s similar to today’s phrase, “Oh my God!” People in the United States have gotten too informal about swearing. These phrases, like the really vulgar or excretory ones used today, were considered to be rude when used in polite company.
~VOW