Etymology
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Rubicon (n.)

in the figurative phrase cross (or pass) the Rubicon "take a decisive step," 1620s, a reference to a small stream to the Adriatic on the coast of northern Italy which in ancient times formed part of the southern boundary of Cisalpine Gaul. It was crossed by Caesar, Jan. 10, 49 B.C.E., when he left his province to attack Pompey. The name is from Latin rubicundus "ruddy," in reference to the color of the soil on its banks.

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coup (n.)
Origin and meaning of coup

c. 1400, "a blow" (obsolete), from Old French coup, colp "a blow, strike" (12c.), from Medieval Latin colpus, from Vulgar Latin *colapus, from Latin colaphus "a cuff, box on the ear," from Greek kolaphos "a blow, buffet, punch, slap," "a lowly word without clear etymology" [Beekes].

Meaning "a sudden decisive act" is 1852, short for coup d'etat. In Modern French the word is a workhorse, describing everything from a pat on the back to a whipping, and is used as well of thunder, gusts of wind, gunshots, and chess moves.

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peremptory (adj.)

mid-15c., peremptorie, "absolute, allowing no refusal," a legal term, from Anglo-French peremptorie, from Late Latin peremptorius "destructive, decisive, final," from peremptor "destroyer," agent noun from past-participle stem of Latin perimpere "destroy, cut off," from per "away entirely, to destruction" (see per) + emere (past participle emptus) "to take" (from PIE root *em- "to take, distribute"). Of persons or their words, "certain, assured, brooking no debate or question," 1580s. Related: Peremptorily.

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sockdolager (n.)
1830, "a decisive blow" (also, figuratively "a conclusive argument"), fanciful formation from sock (v.1) "hit hard," perhaps via a comical mangling of doxology, on a notion of "finality." The meaning "something exceptional" is attested from 1838.

Sockdologising likely was nearly the last word President Abraham Lincoln heard. During the performance of Tom Taylor's "Our American Cousin," assassin John Wilkes Booth (who knew the play well) waited for the laugh-line "Well, I guess I know enough to turn you inside out, you sockdologising old man-trap." Amid the noise as the audience responded, Booth fired the fatal shot.
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obiter dictum 

"statement in passing," a judge's expression of opinion not regarded as binding or decisive, Latin, literally "something said incidentally;" from obiter "by the way" + dictum in the legal sense "a judge's expression of opinion which is not the formal resolution of a case or determination of the court."

Obiter dicta, legal dicta ... uttered by the way (obiter), not upon the point or question pending, as if turning aside for the time from the main topic of the case to collateral subjects. [Century Dictionary]

Latin obiter is from ob "in front of, toward" (see ob-) + iter "journey" (from PIE root *ei- "to go"). Klein's sources, however, say it is ob with the suffix -iter on analogy of circiter "about" from circa. Also see obituary

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die (n.)

"small cube marked on each face with spots numbering from one to six, used in gaming," early 14c. (as a plural, late 14c. as a singular), from Old French de "die, dice," which is of uncertain origin. Common Romanic (cognates: Spanish, Portuguese, Italian dado, Provençal dat, Catalan dau), perhaps from Latin datum "given," past participle of dare "to give" (from PIE root *do- "to give"), which, in addition to "give," had a secondary sense of "to play" (as a chess piece); or else the notion is "what is given" (by chance or Fortune).

The numbers on the opposite sides always add up to seven; otherwise there is no uniformity to their arrangement. Sense of "engraved stamping block or tool used for stamping a softer material" is from 1690s. Perhaps so called because they often were used in pairs (to impress on both sides, as of a coin). Figurative phrase the die is cast "the decisive stem is taken" is from 1630s, in reference to the throw of the dice.

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turn (v.)
late Old English turnian "to rotate, revolve," in part also from Old French torner "to turn away or around; draw aside, cause to turn; change, transform; turn on a lathe" (Modern French tourner), both from Latin tornare "to polish, round off, fashion, turn on a lathe," from tornus "lathe," from Greek tornos "lathe, tool for drawing circles," from PIE root *tere- (1) "to rub, turn." Transitive sense in English is from c. 1300. Related: Turned; turning.

Use in expression to turn (something) into (something else) probably retains the classical sense of "to shape on a lathe." To turn up "arrive, make an appearance" is recorded from 1755. Turn about "by turns, alternately" is recorded from 1640s. To turn (something) loose "set free" is recorded from 1590s. Turn down (v.) "reject" first recorded 1891, American English. Turn in "go to bed" is attested from 1690s, originally nautical. To turn the stomach "nauseate" is recorded from 1620s. To turn up one's nose as an expression of contempt is attested from 1779.

Turning point is attested by 1640s in a figurative sense "point at which a decisive change takes place;" literal sense "point on which a thing turns; point at which motion in one direction ceases and that in another or contrary direction begins" is from 1660s.
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