It is Latin, and means "without release". (unto death) In Rome people fought to the death. The fight was not over until one of them was dead.

The kind of "English" it is, is Latin. Latin phrases are often used in British English, especially when the reader is assumed to have an educated background.

I agree with Eliot. We've lifted the phrase from the Latin, and in cases like this it's assumed the listener/reader will be well-read and therefore have some classical knowledge (ie. know some Latin). How often we do this depends on nearby influences: British English will sooner adopt French phrases from across the channel, and American English - especially in the southern states - will tend to adopt Spanish phrases from its Spanish-speaking neighbours.

one of a kind =)