Prose
The word "prose" first appears in English in the 14th century. It is derived from the Old French "prose", which in turn originates in the Latin expression "prosa oratio" (literally, straightforward or direct speech).
Conspiracy theory
The "Oxford English Dictionary" defines "conspiracy theory" as "the theory that an event or phenomenon occurs as a result of a conspiracy between interested parties; "spec." a belief that some covert but influential agency (typically political in motivation and oppressive in intent) is responsible for an unexplained event". It cites a 1909 article in "The American Historical Review" as the earliest usage example, although it also appears in journals as early as April 1870. The word "conspiracy" derives from the Latin "con-" ("with, together") and "spirare" ("to breathe").