Etymology
Advertisement
post factum 

Latin, literally "after the fact," from post "behind, after, afterward" + factum "deed, act" (see post- + fact).

Related entries & more 
Advertisement
ipso facto 

Latin adverbial phrase, literally "by that very fact, by the fact itself," from neuter ablative of ipse "he, himself, self" + ablative of factum "fact" (see fact).

Related entries & more 
ex post facto 

from Medieval Latin ex postfacto, "from what is done afterwards." From facto, ablative of factum "deed, act" (see fact). Also see ex-, post-.

Related entries & more 
de facto 

Latin, literally "in fact, in reality," thus, "existing, but not necessarily legally ordained or morally right;" from facto, ablative of factum "deed, act" (see de +  fact).

Related entries & more 
au fait (adj.)

1743, French, "to the point, to the matter under discussion," literally "to the fact," from au "to the" (see au) + fait "fact" (see feat). Used in French with sense of "acquainted with the facts, expert, fully skilled."

Related entries & more 
Advertisement
fait accompli (n.)

"a scheme already carried into execution," 19c., French, literally "an accomplished fact." See feat and accomplish.

Related entries & more 
Rh factor 

1942, from the first letters of rhesus; so called because the blood group, and its effects, were discovered in the blood of rhesus monkeys (1941).

Related entries & more 
civil disobedience (n.)

coined 1866 by Thoreau as the title of his essay originally published (1849) as "Resistance to Civil Government."

If the alternative is to keep all just men in prison, or give up war and slavery, the State will not hesitate which to choose. If a thousand men were not to pay their tax-bills this year, that would not be a violent and bloody measure, as it would be to pay them, and enable the State to commit violence and shed innocent blood. This is, in fact, the definition of a peaceable revolution, if any such is possible. [Thoreau]
Related entries & more 
corpus delicti 

1832, Latin, literally "body of the offense;" not "the murder victim's body," but the basic elements that make up a crime, which in the case of a murder includes the body of the victim. For first element, see corpus. With delictum "a fault, offense, crime, transgression," etymologically "a falling short" of the standard of law, neuter singular of past participle of delinquere "to fail; be wanting, fall short; offend" (see delinquent).

Thus, a man who is proved to have clandestinely buried a dead body, no matter how suspicious the circumstances, cannot thereby be convicted of murder, without proof of the corpus delicti--that is, the fact that death was feloniously produced by him. [Century Dictionary]
Related entries & more 
blue moon (n.)

"a long time," 1821, often in phrases indicating something rarely occurring. Compare at the Greek calends (from an ancient Roman phrase alluding to the fact that the Greeks had nothing corresponding to the Roman calends), and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," reported in Grose (1788). Nevermass "date which never comes" is from 1540s. Blue moon is suggested earliest in this couplet from 1528:

Yf they say the mone is blewe,
We must beleve that it is true.

Though this might refer to calendrical calculations by the Church. Thus the general "rareness" sense of the term is difficult to disentangle from the specific calendrical one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation). In either case, the sense of blue here is obscure. Literal blue moons do sometimes occur under extreme atmospheric conditions.

Related entries & more