That is the Latin phrase of "reduction to absurdity", used as a way to show the fallacy of an argument.
I also see a new application of the phrase applying it to physicists who argue that all the other sciences can be reduced to physics. The logic goes that chemistry is all about the electron which can be described by the Schrodinger equation, and since all of biology is chemical reactions, everything can be reduced to physics. (xkcd recently had a slightly different spin on this.) Certainly it can be done, but doing so may actually be counter productive.
This is because humans have some weird quirks in processing information. An example of this is the refrigerator poetry kits - words with magnetic backings that you slide around on the fridge and make up poems. People are always amazed by how nice a poem they can quickly create despite their belief that they aren't a poet. Now imagine the situation if you were to take the words and cut them into individual letters. Most people would freeze and be unable to create much of anything despite the fact that the exact same set of letters is there in both cases.
Phyics: chemistry: biology
Letters: words: paragraphs
Yes, the originally proposed reduction can be completed, but would it be worthwhile at all? Would anything be learned? What would be lost?
Notes: 1) The original idea of cutting up the magnetic words was in an article in Nature from about 10+ years ago. I can't provide any citation. The analysis stopped at the idea that humans don't process all information sets the same even if the same data is present in all cases.
2) This is a (partial) apology to biologists for the title of the last post, "Chemical (Biological) Switching". After all, the examples given were biological in nature, not chemical, something I intend to start changing.