[Vocabulary] on your life, my Lord King

What does "on your life" here mean?
Does it has the same meaning as "not on your life" or does it refer to something else?

“For though I have never been called a coward, I must plainly say that to meet that young man in battle is more than my heart would serve me for. And if (as is likely) his brother, the High King, is more dangerous than he—why, on your life, my Lord King, have nothing to do with him.”



Source: Prince Caspian, by C. S. Lewis

It means 'at the risk of losing your life'.

The young man is an extremely skilled and dangerous fighter. His brother the High King is likely even more dangerous. In fact, so dangerous that he may be a threat even off the battlefield, so avoid him at all cost.

Isn't it a way of goading the KIng? It's a long time since I read it.

I'd need more context to be sure, but he could mean the opposite of what he seems to be saying. In other words, he could be mocking him. (As always, context is everything.)

I don't have a copy to hand, but I think he's trying to make the King look a coward.