Etymology
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siege (n.)
early 13c., "a seat" (as in Siege Perilous, early 13c., the vacant seat at Arthur's Round Table, according to prophecy to be occupied safely only by the knight destined to find the Holy Grail), from Old French sege "seat, throne," from Vulgar Latin *sedicum "seat," from Latin sedere "to sit," from PIE root *sed- (1) "to sit." The military sense is attested from c. 1300; the notion is of an army "sitting down" before a fortress.
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Presidium (n.)

permanent administrative committee of the U.S.S.R., 1924, from Russian prezidium, from Latin praesidium "a presiding over, defense," from praesidere "to protect," literally "to sit before" (see preside).

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settle (n.)
"long bench," 1550s, from Middle English setle "a seat," from Old English setl "a seat, stall; position, abode; setting of a heavenly body," related to sittan "to sit," from Proto-Germanic *setla- (source also of Middle Low German, Middle Dutch setel, Dutch zetel, German Sessel, Gothic sitls), from PIE *sedla- (source also of Latin sella "seat, chair," Old Church Slavonic sedlo "saddle," Old English sadol "saddle"), from root *sed- (1) "to sit."
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session (n.)

late 14c., "periodical sitting of a court," from Old French session "act or state of sitting; assembly," from Latin sessionem (nominative sessio) "act of sitting; a seat; loitering; a session," noun of action from past participle stem of sedere "to sit" (from PIE root *sed- (1) "to sit"). Sense of "period set aside for some activity" is first recorded 1920, in bull session, probably from quarter sessions courts (see quarter (n.1)). Musical sense of "recording occasion in a studio" is from 1927.

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seat (n.1)
"thing to sit on; act of sitting," c. 1200, from Old Norse sæti "seat, position," from Proto-Germanic *sæt- (source also of Old High German saze, Middle Dutch gesaete "seat," Old High German gisazi, German Gesäß "buttocks"), from PIE root *sed- (1) "to sit." Meaning "posterior of the body" (the sitting part) is from c. 1600; sense of "part of a garment which covers the buttocks" is from 1835. Seat belt is from 1915, originally in airplanes.
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-in (1)
the adverb in attached to a verb as a word-forming element, by 1960, abstracted from sit-in, which is attested from 1941 in reference to protests and 1937 in reference to labor union actions (which probably was influenced by sit-down strike) but was popularized in reference to civil disobedience protests aimed at segregated lunch counters. As a word-forming element at first of other types of protests, extended by 1965 to any sort of communal gathering (such as love-in, 1967).
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dodecahedron (n.)

"solid having twelve faces," 1560s, from Greek dōdeka "twelve" (see dodeca-) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit." Related: Dodecahedral.

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assiento (n.)
1714, "contract between the King of Spain and another power," especially that made at the Peace of Utrecht, 1713, with Great Britain for furnishing African slaves to the Spanish colonies in the Americas (abrogated in 1750), from Spanish asiento, formerly assiento "a compact or treaty; a seat in court, a seat," from asentar/assentar "to adjust, settle, establish," literally "to place on a seat," from a sentar, from Latin ad "to" (see ad-) + sedens, present participle of sedere "to sit," from PIE root *sed- (1) "to sit."
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decahedron (n.)

in geometry, "a solid having ten faces," 1828, from deca- "ten" + -hedron, from Greek hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit."  Related: Decahedral.

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perch (v.)

"to roost, to alight or settle on or as on a perch; to occupy some elevated position," late 14c., from Old French perchier "to sit on a perch" (of a bird), from perche (n.); see perch (n.1). Related: Perched; perching.

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