The title of this thread is a bit misleading, I don't mean where biopolitics comes from as much as where the word comes from. (NB, much of this is cribbed shamelessly from Esposito's Bios. You should read that).
Most commonly, it is thought that Michel Foucault coined the term, biopolitics (this is something that even wikipedia gets wrong), this is incorrect.
It was actually coined by the swede, Rudolf Kjellén, in his 1916 book Staten som Lifsform (The State as Form of Life). The same man that coined the term geopolitics.
Kjellén's was one of the more prominent thinkers of a group of German language political theorists; including Friedrich Ratzel, Karl Haushofer, Karl Binding, Eberhard Dennert, and Edward Hahn. What ties these theorists together is first a belief in the organicist nature of the state (the state was a living entity for these thinkers) and the belief in lebensraum (living space). The term lebensraum, originally coined by biologist would get one of it's most sustained treatments under Ratzel, who argued that the German people (the volk) needed a living space. To acquire this living space the German state needed to be responsible for expansion, and also for cutting away the parasitic parts inside the state. Lebensraum is cited by Hitler directly in Mein Kampf, and forms the basis of much of National Socialism. Within this notion of Lebensraum we see the connection between Nazi's imperial ambition tied to its internal fascisms. Indeed, Lebensraum is a borderline concept, bringing inside and outside into a zone of indetermination.
Kjellén radicalizes all of this, bringing geopolitics as being on the same level and totally co-terminus with ethnopolitics. One cannot have a geopolitical vision that is not simultaneously a vision of a particular people. Combined with the thoughts of the other thinkers mentioned earlier, the state, as a form of life, must protect itself. It must cut away the diseased parts, it must exterminate the parasites, it must do all these actions to guarantee its health as a state and the health of its people. This was biopolitics.
Indeed, this biopolitics becomes a completely naturalized in the 1920s essay, Staatsbiologie: Anatomie, Phisiologie, Pathologie des Staates by Jakob von Uexkull (if you can't place where you heard that name before, he is cited frequently by Heidegger, Deleuze and Guattari, and Agamben). Not only does this essay naturalize biopolitics, but also firmly brings it into the discursive economy of pathology. Biopolitics is tied to determining what is, in the word's of Foucault's teacher Georges Canguilhem, the normal and the pathological.
This is not the only use of the term biopolitics predating Foucault's use of the term (there was a distinctly neo-humanist use of the term by many french intellectuals in the early 60s, and also a different use of the term by a group of american intellectuals in the late 60s and early 70s). However, I think it is illuminating and useful to explore these original uses of the term biopolitics (uses that Foucault most certainly had to know of).
Now, someone already brought up to me the issue that this is not the first time that a state has been seen as a living form, indeed is this not the same conception of Hobbes?
One of the major differences is that Hobbes' Leviathan was an escape from nature, a purposefully artificial (and thus, no matter what it was, contestable) compact. The staatsbiologie was actually a return to a naturalism (and thus an uncontestable), a view that sees the organic reality of the state as necessary and universal (kant's two criteria for a priori).