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-in1

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  1. a suffix, occurring in adjectives of Greek and Latin origin, meaning “pertaining to,” and (in nouns thence derived) also imitated in English (coffin; cousin, etc.).
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Origin of -in1

Middle English -in, -ine < Old French < Latin -inus, -ina, -inum < Greek -inos, -inē, -inon

-in2

  1. a noun suffix used in a special manner in chemical and mineralogical nomenclature (glycerin; acetin, etc.). In spelling, usage wavers between -in and -ine. In chemistry a certain distinction of use is attempted, basic substances having the termination -ine rather than -in (aconitine; aniline, etc.), and -in being restricted to certain neutral compounds, glycerides, glucosides, and proteids (albumin; palmitin, etc.), but this distinction is not always observed.
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Origin of -in2

< New Latin -ina. See -ine2

-in3

  1. a suffixal use of the adverb in, extracted from sit-in, forming compound nouns, usually from verbs, referring to organized protests through or in support of the named activity (kneel-in; chain-in; be-in) or, more generally, to any organized social or cultural activity (cook-in; sing-in).
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for -in

-in

suffix forming nouns
  1. indicating a neutral organic compound, including proteins, glucosides, and glyceridesinsulin; digitoxin; tripalmitin
  2. indicating an enzyme in certain nonsystematic namespepsin
  3. indicating a pharmaceutical substancepenicillin; riboflavin; aspirin
  4. indicating a chemical substance in certain nonsystematic namescoumarin
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Word Origin

from New Latin -ina; compare -ine ²
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for -in

1

suffix attached to a verb, first attested 1960 with sit-in (which probably was influenced by sit-down strike); used first of protests, extended c.1965 to any gathering.

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2

chemical suffix, usually indicating a neutral substance, antibiotic, vitamin, or hormone; see -ine (2).

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Online Etymology Dictionary, © 2010 Douglas Harper

-in in Medicine

-in

suff.
  1. Neutral chemical compound:inulin.
  2. Pharmaceutical:rifampin.
  3. Antibiotic:penicillin.
  4. Antigen:tuberculin.
  5. Variant of-ine