"I like the structural path we are on. As long as we help Ukraine with the weapons it needs and economic support, they will fight to the last person." - US Senator Lindsey Graham

To me it seems like such an obvious pull from collective culture that I don't expect there to be like one particular source, it would be funny if it originated from an accident tho.

Not sure of the original but this genius Russian propaganda video certainly mentions it

It’s been said by a few people now.

Putin said it during a meeting with the Security Council of Russia. A similar thing was also said by Neil Hauer, a “journalist who primarily focuses on Russia’s role in the Syrian conflict” who said that the West will fight Russia “to the last drop of Ukrainian blood.”

I searched around a little and the earliest I could find anyone using that exact term is Chas Freeman saying it in March 2022