Many expressions from science have entered our everyday vocabulary, but sometimes in ways that are puzzling. Here are a few that I have wondered about -- perhaps some of you can explain them:
1. Guinea pig. This is now a colloquial expression for any experimental subject. But biomedical research is performed mostly with rats and mice. So instead of saying, "Can you be my guinea pig for this recipe?" why don't we say, "Can you be my rat for this recipe?" Were guinea pigs much more important in research fifty or a hundred years ago? If so, it certainly wasn't for ease of breeding. We used to own a guinea pig, and they do not "breed like rabbits" -- more like pandas. I should note that the term "lab rat" seems to be entering the popular vocabulary, so maybe it will eventually displace "guinea pig."
2. Schizophrenic. In popular usage, "schizophrenic" has come to mean "containing contradictory ideas or elements." While schizophrenia is a serious illness, it is not the same thing as multiple personality disorder, which is exceptionally rare. So how did "schizophrenic" come to be associated with "split personality," and from there with self-contradictory ideas?
3. Finite. "Finite," of course, means "not infinite." So what does it mean when I say "Donald Trump has a finite chance of becoming the Republican nominee"? Surely his chances aren't infinite! What has happened is that "finite" no longer means just "not infinite." It has also taken on the completely opposite meaning of "not zero." (Scientists have adopted this usage as well -- I wrote a recent paper on "Cosmological particle decays at finite temperature", which doesn't mean that the temperate is not infinite -- it means that it's not zero). So how did "finite" come to encompass both its original meaning, and the opposite meaning?
Update: Another example from the comments section: quantum leap. Quantum transitions in atoms are actually very tiny, but they are also discontinuous. So I assume that a "quantum leap" originally referred to a sudden, sharp transition. From there it gradually came to mean a huge change, which is the exact opposite of a real quantum leap.