I have a different take on this: the word defy refers to the entire phrase “until death do us part”. That phrase is part of many marriage vows. You swear to love one another until death separates you. This sentence is saying they could defy that and continue to love one another even after their death.

If you look "defy" up in google, there are a few meanings here's one that might be helpful be of such a kind or nature that (a specified attitude or action) is almost impossible. "his actions defy belief" in my opinion, this means that they love each other, but for some reason they have to resist that feeling(maybe they are both married or something), so much so that even being in each others arms they will have to pretend there's no love between them, and that's what they are going to do until they die hope this helps

i think it means like "going past". or going beyond. but in reality it doesnt really make sense haha you cant really defy love in my opinion since its supposedly the strongest force out there

Thanks for your reply at first. :) I will try to describe this story, but my English is not good, so ... hope you can understand my words. It's about a girl and a boy, they loved each other. But the boy died in an accident, while the girl survived. The girl felt so sad that she hoped she could die with the boy. So she thought: We could have died in each other’s arms, defying love until death do us part. That's all. Sorry for my poor description again. And lots of thanks again.

Where did you find it? It's difficult for us to help you without any context.