Etymology
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is (v.)
third person singular present indicative of be, Old English is, from Germanic stem *es- (source also of Old High German, German, Gothic ist, Old Norse es, er), from PIE *es-ti- (source also of Sanskrit asti, Greek esti, Latin est, Lithuanian esti, Old Church Slavonic jesti), third person singular form of root *es- "to be." Old English lost the final -t-.

Until 1500s, pronounced to rhyme with kiss. Dialectal use for all persons (I is) is in Chaucer. Phrase it is what it is, indicating resigned acceptance of an unpleasant but inevitable situation or circumstance about which nothing truly positive can be said, is attested by 2001.
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isness (n.)
"essence," 1865, in a translation of Hegel, from is + -ness.
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I'se 
1847 in representations of African-American vernacular, a contraction of I is (see is), irregular for I am. In Scottish and northern English, a colloquial or dialectal contraction of I shall (1796).
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entity (n.)

1590s, "being," from Late Latin entitatem (nominative entitas), from ens (genitive entis) "a thing," proposed by Caesar as present participle of esse "be" (see is), to render Greek philosophical term to on "that which is" (from neuter of present participle of einai "to be," from PIE root *es- "to be"). Originally abstract; concrete sense in English is from 1620s.

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id est 
Latin, literally "that is (to say)," from id "that," neuter of is, from PIE pronominal stem *i- (see yon). For est, see is. Usually abbreviated i.e. "to write, or even to say, this in the full instead of in the abbreviated form is now so unusual as to convict one of affectation" [Fowler]. It introduces another way to say something already said, not an example of it (which is e.g.).
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i.e. 

abbreviation of Latin id est, literally "that is;" used in English in the sense of "that is to say." Latin id "it" is from PIE pronominal stem *i- (see yon). For est, see is.

i.e. means that is to say, & introduces another way (more comprehensible to the hearer, driving home the speaker's point better, or otherwise preferable) of putting what has been already said; it does not introduce an example, & when substituted for e.g. in that function ... is a blunder. [Fowler]
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Isidore 

masc. proper name, from French, from Latin Isidorus, from Greek Isidoros, literally "gift of Isis," from Isis (see Isis) + dōron "gift" (from PIE root *do- "to give"). St. Isidore, archbishop of Seville (600-636) wrote important historical, etymological, and ecclesiastical works and in 2001 was named patron saint of computers, computer users, and the internet. Related: Isidorian.

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isinglass (n.)
purest commercial form of gelatin, 1520s, apparently a perversion of Dutch huysenblas, literally "sturgeon bladder," from huysen "sturgeon" + blas "bladder," from Proto-Germanic *bles-, extended form of PIE root *bhle- "to blow." So called because the substance was obtained from the air-bladders of certain freshwater fishes.
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isosceles (adj.)

"having two equal sides," 1550s, from Late Latin isosceles, from Greek isoskeles "with equal legs; isosceles; that can be divided into two equal parts," from isos "equal, identical" (see iso-) + skelos "leg," from PIE *skel-es-, from root *skel- "bend, curve" (see scoliosis).

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-istic 
adjectival word-forming element, from French -istique or directly from Latin -isticus, from Greek -istikos, a compound of the adjectival suffix -ikos (see -ic) + the noun suffix -istes (see -ist).
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