Traditionally a blood oath was when one would shed their own blood(Cut yourself or something) and offer it onto a altar or whatever they believed right(sometimes a religious action) and swore to uphold a certain task, no matter what. Also it was when a person(s) would shed blood, usually they'd cut their hand and then shake hands in agreement to a task they swear to uphold.

More modernly it just means you swear to do something no matter what, and since it's a blood oath, modernly it means hurting or even killing others in oath, thus 'blood' oath, meaning blood would be shed in promise to the task, usually a phrase only used by mobsters and crooks.
The great samuraiKenji Setsuna bowed before the altar and cut his hand, offering blood, swearing to protect his people under any condition in belief before his God.

The Prince and his loyal Knight cut their palms and then shook hands, promising each other in a blood oath they'd never split apart.

Officer Eliot of the Chicago PD & Officer Hunter of the Seattle PD made a blood oath that whatever happens, they will stop crime in their city, willing to do whatever it takes to keep this oath.

Kingpin Chuck Norris told a footman of his black market empire that if he did not go through with the hit, he'd kill his family. The footman then made a blood oath, swearing he would.
by Zure the Dark Being April 12, 2013
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When a male with gingivitis/gum disease preforms oral sex on his sister when she's menstruating.
Billy and his sister took an Alabama Blood Oath last night.
by FkcdOffCO July 12, 2022
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Eating KFC is a delicious extravaganza. A mouth watering joy of essence itself. But the Kernel will take back what it’s due. One may eat of its tastiness now, but the one will suffer from severe fire poops that draw blood shortly after. And this is the KFC Blood Oath.
Hey Josh, you eat that delicious KFC yet?

Sure have man! I enjoyed every moment!

Yeah, I’m sure! You think the KFC Blood Oath will be repaid tonight, or tomorrow morning?

Oh tonight for sure!!
by the teli May 25, 2022
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