by 1766 as a Southern U.S. derogatory term for "one of an inferior class of white hill-dwellers in some of the southern United States" [Century Dictionary], probably an agent noun from crack (v.) in its sense "to boast" (as in not what it's cracked up to be).

Cracker as "a boaster, a braggart" is attested from mid-15c. ("Schakare, or craker, or booste maker: Jactator, philocompus," in Promptorium Parvulorum, an English-Latin dictionary); also see crack (n.). It also was a colloquial word for "a boast, a lie" (1620s). For sense development, compare Latin crepare "to rattle, crack, creak," with a secondary figurative sense of "boast of, prattle, make ado about." This also was the old explanation of the term:

I should explain to your Lordship what is meant by crackers; a name they have got from being great boasters; they are a lawless set of rascalls on the frontiers of Virginia, Maryland, the Carolinas and Georgia, who often change their places of abode. [letter from colonial officer Gavin Cochrane to the Earl of Dartmouth, June 27, 1766]

For an alternative, DARE compares corn-cracker "Kentuckian," also "poor, low-class white farmer of Georgia and North Carolina" (1835, U.S. Midwest colloquial).

The word was used especially of Georgians by 1808, though often extended to residents of northern Florida. Another name in mid-19c. use was sand-hiller "poor white in Georgia or South Carolina."

Not very essentially different is the condition of a class of people living in the pine-barrens nearest the coast [of South Carolina], as described to me by a rice-planter. They seldom have any meat, he said, except they steal hogs, which belong to the planters, or their negroes, and their chief diet is rice and milk. "They are small, gaunt, and cadaverous, and their skin is just the color of the sand-hills they live on. They are quite incapable of applying themselves steadily to any labor, and their habits are very much like those of the old Indians." [Frederick Law Olmsted, "A Journey in the Seaboard Slave States," 1856]

Old English deofol "a devil, a subordinate evil spirit afflicting humans;" also, in Christian theology, "the Devil, a powerful spirit of evil otherwise known as Satan," from Late Latin diabolus (also the source of Italian diavolo, French diable, Spanish diablo; German Teufel is Old High German tiufal, from Latin via Gothic diabaulus).

The Late Latin word is from Ecclesiastical Greek diabolos, which in Jewish and Christian use was "the Devil, Satan," and which in general use meant "accuser, slanderer" (thus it was a scriptural loan-translation of Hebrew satan; see Satan). It is an agent noun from Greek diaballein "to slander, attack," literally "to throw across," from dia "across, through" (see dia-) + ballein "to throw" (from PIE root *gwele- "to throw, reach").

Jerome re-introduced Satan in Latin bibles, and English translators have used both words in different measures. In Vulgate, as in Greek, diabolus and dæmon (see demon) were distinct, but they have merged in English and other Germanic languages.

Meaning "false god, heathen god" is from c. 1200. Sense of "diabolical person, person resembling a devil or demon in character" is from late 12c. Playful use for "clever rogue" is from c. 1600. As an expletive and in expletive phrases from c. 1200.

Meaning "sand spout, dust storm" is from 1835 (dust devil is attested by 1867). In U.S. place names, the word often represents a native word such as Algonquian manito, more properly "spirit, god." Phrase a devil way (c. 1300) was originally "Hell-ward, to Hell," but by late 14c. it was a mere expression of irritation. Meaning "errand-boy in a printing office" is from 1680s, perhaps because they were often blackened by the ink (devils then being popularly supposed to be black).

Devil's books "playing cards" is from 1729, but the cited quote says they've been called that "time out of mind" (the four of clubs is the devil's bedposts); devil's coach-horse is from 1840, the large rove-beetle, which is defiant when disturbed. Devil's food cake (1895; three different recipes in the cookbook "compiled by the Ladies' Aid Society of the Friends' Church, Wilmington, Ohio"), rich and chocolate, probably is in deliberate contrast to angel food cake. Conventional phrase talk (or speak)of the Devil, and he's presently at your elbow is by 1660s.

Old English lætan (Northumbrian leta) "to allow; to leave behind, depart from; leave undone; bequeath," also "to rent, put to rent or hire" (class VII strong verb; past tense let, leort, past participle gelæten), from Proto-Germanic *letan (source also of Old Saxon latan, Old Frisian leta, Dutch laten, Old High German lazan, German lassen, Gothic letan "to leave, let"), from PIE *led-, extended form of root *‌‌lē- "to let go, slacken." If that derivation is correct, the etymological sense might be "let go through weariness, neglect."

The idea of slackening lies at the root of both applications of the term. When we speak of letting one go, letting him do something, we conceive him as previously restrained by a band, the loosening or slackening of which will permit the execution of the act in question. ... At other times the slackness is attributed to the agent himself, when let acquires the sense of being slack in action, delaying or omitting to do. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859] 

He points to similar developments in French laisser "to let" from Latin laxare "to slacken," German lassen "to permit, to let," from dialectal lass "loose."

"The shortening of the root vowel ... has not been satisfactorily explained" [OED]. Of blood, from late Old English. Other Old and Middle English senses include "regard as, consider; behave toward; allow to escape; pretend;" to let (someone) know and to let fly (arrows, etc.) preserve the otherwise obsolete sense of "to cause to."

To let (someone) off "allow to go unpunished, excuse from service" is from 1814. To let on is from 1725 as "allow (something) to be known, betray one's knowledge of," 1822 as "pretend" (OED finds a similar use in the phrase never let it on him in a letter from 1637). To let out is late 12c. as "allow to depart" (transitive); intransitive use "be concluded," of schools, meetings, etc., is from 1888, considered by Century Dictionary (1895) to be "Rural, U.S." Of garments, etc., late 14c.

Let alone "abstain from interfering with" is in Old English; the phrase in the sense "not to mention, to say nothing of" is from 1812. To let (something) be "leave it alone" is from c. 1300; let it be "let it pass, leave it alone" is from early 14c. To let go is from c. 1300 as "allow to escape," 1520s as "cease to restrain," 1530s as "dismiss from one's thoughts." Let it go "let it pass, no matter" is as old as Chaucer's Wife of Bath: "But age allas Hath me biraft my beautee Lat it go, far wel, the deuel go ther with!" [c. 1395]. Let me see "show me" is from c. 1300.

a Middle English merger of Old English line "cable, rope; series, row, row of letters; rule, direction," and Old French ligne "guideline, cord, string; lineage, descent" (12c.), both from Latin linea "linen thread, string, plumb-line," also "a mark, bound, limit, goal; line of descent," short for linea restis "linen cord," and similar phrases, from fem. of lineus (adj.) "of linen," from linum "linen" (see linen).

The earliest sense in Middle English was "cord used by builders for taking measurements;" extended late 14c. to "a thread-like mark" (from sense "cord used by builders for making things level," mid-14c.), also "track, course, direction." Meaning "limit, boundary" (of a county, etc.) is from 1590s. The mathematical sense of "length without breadth" is from 1550s. From 1530s as "a crease of the face or palm of the hand." From 1580s as "the equator."

Sense of "things or people arranged in a straight line" is from 1550s. Now considered American English, where British English uses queue (n.), but the sense appears earliest in English writers. Sense of "chronologically continuous series of persons" (a line of kings, etc.) is from late 14c.

Meaning "one's occupation, branch of business" is from 1630s, according to OED probably from misunderstood KJV translation of II Corinthians x.16, "And not to boast in another mans line of things made ready to our hand," where line translates Greek kanon which probably meant "boundary, limit;" the phrase "in another man's line" being parenthetical.

Commercial meaning "class of goods in stock" is from 1930, so called from being goods received by the merchant on a line in the specific sense "order given to an agent" for particular goods (1834). Insurance underwriting sense is from 1899. Line of credit is from 1958.

Meaning "series of public conveyances" (coaches, later ships) is from 1786; meaning "continuous part of a railroad" is from 1825. Meaning "telegraph wire between stations" is from 1847 (later "telephone wire"). Meaning "cord bearing hooks used in fishing" is from c. 1300. Meaning "policy or set of policies of a political faction" is 1892, American English, from notion of a procession of followers; this is the sense in the political party line, and, deteriorated, it is the slang line that means "glib and plausible talk meant to deceive."

In British army, the Line (1802) is the regular, numbered troops, as distinguished from guards, auxiliaries, militia, etc. In the Navy (1704) it refers to the battle line (the sense in ship of the line, which is attested from 1706).

Dutch lijn, Old High German lina, German Leine, Old Norse lina "a cord, rope," are likewise from Latin. Spanish and Italian have the word in the learned form linea. In continental measurements, a subdivision of an inch (one-tenth or one-twelfth in England), attested in English from 1660s but never common. Also see lines.

To get a line on "acquire information about" is from 1903. To lay it on the line is from 1929 as "to pay money;" by 1954 as "speak plainly." End of the line "as far as one can go" is from 1948. One's line of work, meaning "pursuit, interest" is from 1957, earlier line of country (1861). Line-drawing is from 1891. A line-storm (1850) is a type supposed to happen in the 10 days or two weeks around the times the sun crosses the equator.