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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.).
Origin of -in1
Middle English -in, -ine < Old French < Latin -inus, -ina, -inum < Greek -inos, -inē, -inon


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.
< New Latin -ina. See -ine2


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). Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for -in


indicating a neutral organic compound, including proteins, glucosides, and glycerides: insulin, digitoxin, tripalmitin
indicating an enzyme in certain nonsystematic names: pepsin
indicating a pharmaceutical substance: penicillin, riboflavin, aspirin
indicating a chemical substance in certain nonsystematic names: coumarin
Word Origin
from New Latin -ina; compare -ine²
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
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Word Origin and History for -in

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.


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

Online Etymology Dictionary, © 2010 Douglas Harper
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-in in Medicine

-in suff.

  1. Neutral chemical compound: inulin.

  2. Pharmaceutical: rifampin.

  3. Antibiotic: penicillin.

  4. Antigen: tuberculin.

  5. Variant of -ine2.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Slang definitions & phrases for -in


combining word

A communal occasion where one does what is indicated: be-in/ lie-in/ love-in/ pray-in (1960s+)

The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D.
Copyright (C) 2007 by HarperCollins Publishers.
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