Etymology
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its (pron.)
neuter possessive pronoun; late 16c., from it + genitive/possessive ending 's (q.v.). "[A]t first commonly written it's, a spelling retained by some to the beginning of the 19c." [OED]. The apostrophe came to be omitted, perhaps because it's already was established as a contraction of it is, or by general habit of omitting apostrophes in personal pronouns (hers, yours, theirs, etc.).

The neuter genitive pronoun in Middle English was his, but the clash between grammatical gender and sexual gender, or else the application of the word to both human and non-human subjects, evidently made users uncomfortable. Restriction of his to the masculine and avoidance of it as a neuter pronoun is evidenced in Middle English, and of it and thereof (as in KJV) were used for the neuter possessive. In literary use, his as a neuter pronoun continued into the 17c. In Middle English, simple it sometimes was used as a neuter possessive pronoun (c. 1300).
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his (pron.)
Old English his (genitive of he), from Proto-Germanic *hisa (source also of Gothic is, Old Saxon is, German es). Originally also the neuter possessive pronoun, but in English it was replaced in that sense c. 1600 by its. In Middle English, hisis was tried for the absolute pronoun (compare her/hers), but it failed to stick. For dialectal his'n, see her.

In 16c.-17c. commonly used in place of a genitive inflection after nouns whose nominative ends in -s (for example, "When this Book became a particular book, that is, when Moses his book was divided into five parts, I cannot trace." [Donne, "Essayes in Divinity," "Exodus," 1651]). Here it is perhaps an expanded vocalized form of 's, originally -es. This tendency began in late Old English and was obsolete from c. 1750.
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itself (pron.)
late 14c., from Old English hit sylf, from it + self. Since 17c. usually regarded as its self (thus its own self, etc.).
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chicken pox (n.)

c. 1730, from chicken (n.) + pox. Perhaps so called for its mildness compared to smallpox [Barnhart], or its generally appearing in children, or its resemblance to chick-peas.

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itsy-bitsy (adj.)

"charmingly small," 1882, from itty (baby-talk form of little) and/or bitsy. Bitsy-itsy is recorded from 1875. Itty-bitty from 1849; tiddy-itty from 1852.

The fond old captain used to talk of her very much as if she were six months old, and fractions in teething, calling her his Peetums-keetums, his Pitsy-itsy-kitsy and such like, which annoyed the girl, .... ["M.B.," "John's Wife," in "Tales of the Day," 1861]
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in-itselfness (n.)
1879, in philosophy; see in (adv.) + itself + -ness.
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orotund (adj.)

in elocution, "characterized by strength, fullness, richness, and clearness," 1792, from Latin ore rotundo "in well-rounded phrases," literally "with round mouth" (see ore rotundo).

The odd thing about the word is that its only currency, at least in its non-technical sense, is among those who should most abhor it, the people of sufficient education to realize its bad formation; it is at once a monstrosity in its form & a pedantry in its use. [Fowler]
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hedgehog (n.)
mid-15c. (replacing Old English igl), from hedge (n.) + hog (n.). First element from its frequenting hedges; the second element a reference to its pig-like snout.
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durian (n.)

globular fruit of a tree of Indonesia, 1580s, from Malay (Austronesian) durian, from duri "thorn, prickle." So called for its rind.

The durian is deemed by the Siamese the king of fruits. Its smell is offensive to European sense, and I have heard it compared to the stink of carrion and onions mingled. But the exquisite flavour of the fruit renders even its fragrance attractive to its habitués, and it is the only fruit which has ever a considerable money-value in the Siamese market. [Sir John Bowring, "The Kingdom and People of Siam," London, 1857]
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