Etymology
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manifest (adj.)
Origin and meaning of manifest

late 14c., "clearly revealed to the eye or the understanding, open to view or comprehension," from Old French manifest "evident, palpable," (12c.), or directly from Latin manifestus "plainly apprehensible, clear, apparent, evident;" of offenses, "proved by direct evidence;" of offenders, "caught in the act," probably from manus "hand" (from PIE root *man- (2) "hand") + -festus, which apparently is identical to the second element of infest.

De Vaan writes, "If manifestus may be interpreted as 'caught by hand', the meanings seem to point to 'grabbing' or 'attacking' for -festus." But he finds none of the proposed ulterior connections compelling, and concludes that, regarding infestus and manifestus, "maybe the two must be separated." If not, the sense development might be from "caught by hand" to "in hand, palpable." 

Manifest destiny, "that which clearly appears destined to come to pass; a future state, condition, or event which can be foreseen with certainty, or is regarded as inevitable" was much used in American politics from about the time of the Mexican War "by those who believed that the United States were destined in time to occupy the entire continent" [Century Dictionary].

Other nations have tried to check ... the fulfillment of our manifest destiny to overspread the Continent allotted by Providence for the free development of our yearly multiplying millions. [John O'Sullivan (1813-1895), "U.S. Magazine & Democratic Review," July 1845]

The phrase apparently is O'Sullivan's coinage; the notion is as old as the republic.

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security (n.)

early 15c., securite, "state or condition of being safe from danger or harm;" mid-15c., "freedom from care or anxiety" (a sense now archaic), from Old French securite and directly from Latin securitas "freedom from care," from securus "free from care" (see secure (adj.)).

This form replaced the earlier sikerte (early 15c.), which represents an earlier borrowing of the Latin word; earlier in English in the sense of "security" was sikerhede (early 13c.); sikernesse (c. 1200). Sir Thomas Browne uses securement; Francis Bacon and Mrs. Browning have secureness. Surety is a doublet, via French.

The meaning "something which secures, that which makes safe" is from 1580s. The specific legal sense of "something pledged as a guarantee of fulfillment of an obligation" is from mid-15c. (originally a guarantee of good behavior).

The meaning "safety of a state, person, etc." is by 1941. By 1965 Security, with the capital, was generic or shorthand for "security officials; a state's security department or ministry."

The legal sense of "property in bonds" is from mid-15c.; that of "document held by a creditor as evidence of debt or property and proof of right to payment" is from 1680s. Security-check (n.) is by 1945. Phrase security blanket in figurative sense is attested by 1966, in reference to the crib blanket carried by the character Linus in the popular "Peanuts" newspaper comic strip (the blanket, and the strip, from 1956).

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prove (v.)

c. 1200, prēven, pruven, proven "to try by experience or by a test or standard; evaluate; demonstrate in practice," from Old French prover, pruver "show; convince; put to the test" (11c., Modern French prouver), from Latin probare "to make good; esteem, represent as good; make credible, show, demonstrate; test, inspect; judge by trial" (source also of Spanish probar, Italian probare, and English probe), from probus "worthy, good, upright, virtuous."

This is from PIE *pro-bhwo- "being in front," from *pro-, extended form of root *per- (1) "forward," hence "in front of," + root *bhu- "to be," source also of Latin fui "I have been," futurus "about to be;" Old English beon "to be;" see be.

From early 13c. as "render certain, put out of doubt," also "establish the validity or authenticity of a will, etc." By c. 1300 as "test and find worthy, virtuous, false, etc.," also "find out, discover, ascertain; prove by argument." By mid-14c. as "check the accuracy of." The meaning "be found to be (a hero, coward, etc.) by experience or trial" is by late 14c.

The word had many more senses and broader application in Middle English than Modern English: "to experience; to strive, endeavor; act, accomplish; thrive, succeed." Also in Middle English in a now-obsolete sense of "approve, sanction, praise" (c. 1300; compare approve). Related: Proved; proven; proving. Proving ground "place used for firing cannons for making ballistics tests and testing powder" is by 1837.

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monitor (n.)

1540s, "senior pupil at a school charged with keeping order, etc.," from Latin monitor "one who reminds, admonishes, or checks," also "an overseer, instructor, guide, teacher," agent noun from monere "to remind, bring to (one's) recollection, tell (of); admonish, advise, warn, instruct, teach," from PIE *moneie- "to make think of, remind" (source also of Sanskrit manayati "to honor, respect," Old Avestan manaiia- "making think"), suffixed (causative) form of root *men- (1) "to think" (source also of Latin memini "I remember, I am mindful of," mens "mind") The notion is "one who or that which warns of faults or informs of duties."

The type of lizard (1826) was so called because it is fabled to give warning to man of Nile crocodiles. Meaning "squat, slow-moving type of ironclad warship" (1862) is from the name of the first vessel of this design, chosen by the inventor, Swedish-born U.S. engineer John Ericsson (1803-1889), because it was meant to "admonish" the Confederate leaders in the U.S. Civil War.

I now submit for your approbation a name for the floating battery at Green Point. The impregnable and aggressive character of this structure will admonish the leaders of the Southern Rebellion that the batteries on the banks of their rivers will no longer present barriers to the entrance of the Union forces. The iron-clad intruder will thus prove a severe monitor to those leaders. ... "Downing Street" will hardly view with indifference this last "Yankee notion," this monitor. ... On these and many similar grounds I propose to name the new battery Monitor. [Ericsson to Asst. Sec. of Navy, Jan. 20, 1862]

 Broadcasting sense of "a device to continuously check on the technical quality of a transmission" (1931) led to special sense of "a TV screen displaying the picture from a particular camera."

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