Etymology
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worse (adj.)
Old English wiersa, wyrsa "worse," from Proto-Germanic *wers-izon- (source also of Old Saxon wirs, Old Norse verri, Swedish värre, Old Frisian wirra, Old High German wirsiro, Gothic wairsiza "worse"), comparative of PIE *wers- (1) "to confuse, mix up" (source also of Old High German werra "strife," Old Saxon werran "to entangle, compound;" see war (n.)). Used as a comparative of bad, evil, ill or as the opposite of better. The adverb is Old English wyrs; the noun is Old English wyrsa. Phrase for better or for worse is attested from late 14c. (for bet, for wers); to change for the worse is recorded from c. 1400.
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worser (adj.)
double comparative; see worse + -er (2). Attested from late 15c. and common 16c.-17c. Noun worsers "(one's) inferiors" is from 1580s.
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worsen (v.)
mid-13c., wersnen "to make worse," also "to grow worse," from worse (adj.) + -en (1). The reflexive sense of "to get worse, become worse off" was elevated into literary use c. 1800-30, where formerly worse (v.) had served. Related: Worsened; worsening.
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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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deteriorate (v.)

1640s, transitive, "make worse, reduce in quality," from Late Latin deterioratus, past participle of deteriorare "get worse; make worse," from Latin deterior "worse, lower, inferior, meaner," contrastive of *deter "bad, lower," from PIE *de-tero-, from demonstrative stem *de- (see de). Intransitive sense "grow worse, degenerate" is by 1758. Related: Deteriorated; deteriorating.

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deterioration (n.)

"a growing or making worse," 1650s, possibly a native formation, or else from French détérioration (15c.), noun of action from détériorer, from Late Latin deteriorare "get worse; make worse" (see deteriorate).

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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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pejoration (n.)

"deterioration, a becoming worse," 1650s, noun of action from pejorate (1640s), from Late Latin peiorare "make worse," from Latin peior "worse," perhaps originally "stumbling," from PIE *ped-yos-, suffixed (comparative) form of *ped- "to walk, stumble, impair," from root *ped- "foot." Meaning "a lowering or deterioration of the sense of a word" is by 1889.

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impair (v.)
late 14c., a re-Latinizing of earlier ampayre, apeyre "make worse, cause to deteriorate" (c. 1300), from Old French empeirier "make worse" (Modern French empirer), from Vulgar Latin *impeiorare "make worse," from assimilated form of in- "into, in" (from PIE root *en "in") + Late Latin peiorare "make worse," from peior "worse," perhaps originally "stumbling," from PIE *ped-yos-, suffixed (comparative) of *ped- "to walk, stumble, impair," from root *ped- "foot. In reference to driving under the influence of alcohol, first recorded 1951 in Canadian English. Related: Impaired; impairing.
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pejorative (adj.)

"depreciative, disparaging, giving a low or bad sense to," 1888, from French péjoratif, from Late Latin peiorat-, past-participle stem of peiorare "make worse," from Latin peior "worse," perhaps originally "stumbling," from PIE *ped-yos-, suffixed (comparative) form of *ped- "to walk, stumble, impair," from root *ped- "foot." As a noun, "a word that depreciates the sense," from 1882. English had a verb pejorate "to worsen" from 1640s.

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