Etymology
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sedation (n.)

early 15c., "alleviation of pain;" 1540s, "act of making calm," from French sédation and directly from Latin sedationem (nominative sedatio) "a quieting, assuaging, a calming," noun of action from past-participle stem of sedare (see sedate (adj.)).

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sedative (adj.)
"tending to calm or soothe," early 15c., from Medieval Latin sedativus "calming, allaying," from sedat-, past participle stem of sedare, causative of sedere "to sit," from PIE root *sed- (1) "to sit." The noun derivative meaning "a sedative drug" is attested from 1785. Hence, "whatever soothes or allays."
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sedentary (adj.)

1590s, "remaining in one place," from French sédentaire (16c.) and directly from Latin sedentarius "sitting, remaining in one place," from sedentem (nominative sedens), present participle of sedere "to sit; occupy an official seat, preside; sit still, remain; be fixed or settled," from PIE root *sed- (1) "to sit." Of persons, the sense "not in the habit of exercise" is recorded from 1660s.

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Seder (n.)
home service on the first nights of Passover, 1865, from Hebrew sedher "order, procedure," related to sedherah "row, rank."
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sederunt (n.)
"sitting, session," Latin, literally "they sat" (typical opening word in recordings of such proceedings), third person plural past tense of sedere "to sit," from PIE root *sed- (1) "to sit."
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sedge (n.)
"coarse grass-like plant growing in wet places," Old English secg "sedge, reed, rush," from Proto-Germanic *sagjaz "plant with a cutting edge" (source also of Low German segge, German Segge), from suffixed form of PIE root *sek- "to cut," on notion of plant with "cutting" leaves. Compare Old English secg, identical in form but meaning "sword;" and German schwertel-gras "sedge" from schwert "sword," also see the etymological sense of gladiolus). Old Irish seisg, Welsh hesgreed "rush" might represent a similar sense development from the same root. Often spelled seg, segg until present form triumphed early 1900s.
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sediment (n.)

1540s, "matter which settles at the bottom of water or other liquid," from French sédiment (16c.) and directly from Latin sedimentum "a settling, sinking down," from stem of sedere "to settle, sit" (from PIE root *sed- (1) "to sit").

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sedimentary (adj.)
1764, from sediment + -ary. Sedimentary rock attested by 1830 (in Lyell). Sedimental (adj.) is recorded from c. 1600.
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sedimentation (n.)
1845, from sediment + -ation, ending used in forming nouns of action.
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sedition (n.)

mid-14c., "rebellion, uprising, revolt, concerted attempt to overthrow civil authority; violent strife between factions, civil or religious disorder, riot; rebelliousness against authority," from Old French sedicion (14c., Modern French sédition) and directly from Latin seditionem (nominative seditio) "civil disorder, dissension, strife; rebellion, mutiny," literally "a going apart, separation," from se- "apart" (see secret (n.)) + itio "a going," from ire "to go" (from PIE root *ei- "to go").

Meaning "conduct or language inciting to rebellion against a lawful government" is from 1838. An Old English word for it was folcslite. Less serious than treason, as wanting an overt act, "But it is not essential to the offense of sedition that it threaten the very existence of the state or its authority in its entire extent" [Century Dictionary].

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