Etymology
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worst (adj.)
Old English wyrresta, from Proto-Germanic *wers-ista- (source also of Old Saxon wirsista, Old Norse verstr, Old Frisian wersta, Old High German wirsisto), superlative of PIE *wers- (1) "to confuse, mix up" (see war (n.)). Phrase in the worst way (1839) is from American English sense of "most severely." The adverb is Old English wyrst; the noun, "that which is most evil or bad," is from late 14c.
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worst (v.)
"damage, inflict loss upon," c. 1600, from worst (adj.). Meaning "defeat in argument" is from 1650s. Related: Worsted; worsting.
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bad (adj.)

c. 1300, "inadequate, unsatisfactory, worthless; unfortunate;" late 14c., "wicked, evil, vicious; counterfeit;" from 13c. in surnames (William Badde, Petri Badde, Asketinus Baddecheese, Rads Badinteheved). Rare before 1400, and evil was more common until c. 1700 as the ordinary antithesis of good. It has no apparent relatives in other languages.* Possibly from Old English derogatory term bæddel and its diminutive bædling "effeminate man, hermaphrodite, pederast," which probably are related to bædan "to defile."

The orig. word, AS. bæddel, ME. baddel, on account of its sinister import, is scarcely found in literature, but, like other words of similar sense, it prob. flourished in vulgar speech as an indefinite term of abuse, and at length, divested of its original meaning, emerged in literary use as a mere adj., badde, equiv. to the older evil. [Century Dictionary, 1897]

Comparable words in the other Indo-European languages tend to have grown from descriptions of specific qualities, such as "ugly," "defective," "weak," "faithless," "impudent," "crooked," "filthy" (such as Greek kakos, probably from the word for "excrement;" Russian plochoj, related to Old Church Slavonic plachu "wavering, timid;" Persian gast, Old Persian gasta-, related to gand "stench;" German schlecht, originally "level, straight, smooth," whence "simple, ordinary," then "bad").

Comparative and superlative forms badder, baddest were common 14c.-18c. and used as recently as Defoe (but not by Shakespeare), but yielded to comparative worse and superlative worst (which had belonged to evil and ill).

Meaning "uncomfortable, sorry" is 1839, American English colloquial. To go bad "putrefy" is from 1884. Not bad "fairly good" is by 1771. Ironic use as a word of approval is said to be at least since 1890s orally, originally in African-American vernacular, emerging in print 1928 in a jazz context. It might have emerged from the ambivalence of expressions like bad nigger, used as a term of reproach by whites, but among blacks sometimes representing one who stood up to injustice, but in the U.S. West bad man also had a certain ambivalence:

These are the men who do most of the killing in frontier communities, yet it is a noteworthy fact that the men who are killed generally deserve their fate. [Farmer and Henley, "Slang and Its Analogues"]

*Persian has bad in more or less the same sense as the English word, but this is regarded by linguists as a coincidence. The forms of the words diverge as they are traced back in time (Persian bad comes from Middle Persian vat), and such accidental convergences exist across many languages, given the vast number of words in each and the limited range of sounds humans can make to signify them. Among other coincidental matches with English are Korean mani "many," Chinese pei "pay," Nahuatl (Aztecan) huel "well," Maya hol "hole."

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pits (n.)
"the worst," by 1953, U.S. slang, said to be a shortened form of armpits.
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misanthropy (n.)

"hatred or dislike of mankind, the habit of taking the worst possible view of human character and motives," 1650s, from Greek misanthrōpia "hatred of mankind," from misanthrōpos "hating mankind" (see misanthrope).

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kakistocracy (n.)
"government by the worst element of a society," 1829, coined (by Thomas Love Peacock) on analogy of its opposite, aristocracy, from Greek kakistos "worst," superlative of kakos "bad" (which perhaps is related to PIE root *kakka- "to defecate") + -cracy. Perhaps the closest word in ancient Greek was kakonomia "a bad system of laws and government," hence kakonomos "with bad laws, ill-governed."
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dysgenics (n.)

 "study of the factors producing genetic deterioration, also loosely, "the carrying on of the species by the worst members," 1906, from dys- + ending from eugenics. Hence dysgenic "having or causing a detrimental effect on the race" (1909).

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pis aller (n.)
"last resource, what one would do at the worst," 1670s, French, literally "to go worse," from pis "worse," from Latin peius, neuter of peior "worse" (see pejorative) + aller "to go" (see alley (n.1)).
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pessimism (n.)

1794 "worst condition possible, point of greatest deterioration" (a sense now rare or obsolete), borrowed (by Coleridge) from French pessimisme, formed (on model of French optimisme) from Latin pessimus "worst," perhaps originally "bottom-most," from PIE *ped-samo-, suffixed (superlative) form of *ped- "to walk, stumble, impair," from root *ped- "foot." Compare Latin pessum "downward, to the ground."

As a name given to the metaphysical doctrines of Schopenhauer, Hartmann, etc., that this is the worst possible world, or that everything tends toward evil, it is recorded in English by 1835, from German pessimismus (Schopenhauer, 1819). As "tendency to exaggerate in thought the evils of life or to look only on the dark side," by 1815. The attempt to make a verb of it as pessimize (1862) did not succeed.

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brunt (n.)
late 14c., "a sharp blow," of uncertain origin, perhaps from Old Norse brundr "sexual heat," or bruna "to advance like wildfire" (said of a ship under sail, etc.). Meaning "chief force, the heaviest or worst (of something)," as in bear the brunt, is from early 15c.
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