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in-1

  1. a prefix representing English in (income; indwelling; inland, etc.), but used also as a verb-formative with transitive, intensive, or sometimes little apparent force (intrust; inweave, etc.). It often assumes the same forms as in-2, such as en-1, em-1, im-3.
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Origin of in-1

Middle English, Old English; see in

in-2

  1. a prefix of Latin origin meaning primarily “in,” but used also as a verb-formative with the same force as in-1 (incarcerate; incantation).
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Also il-, im-, ir-.
Compare em-1, en-1.

Origin of in-2

< Latin, combining form of in (preposition); cognate with in

in-3

  1. a prefix of Latin origin, corresponding to English un-, having a negative or privative force, freely used as an English formative, especially of adjectives and their derivatives and of nouns (inattention; indefensible; inexpensive; inorganic; invariable). It assumes the same phonetic phases as in-2 (impartial; immeasurable; illiterate; irregular, etc.). In French, it became en- and thus occurs unfelt in such words as enemy (French ennemi, Latin inimicus, lit., not friendly).
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Also il-, im-, ir-.

Origin of in-3

< Latin; akin to an-1, a-6, un-1

Synonym study

The prefixes in- and un- may both have, among other uses, a negative force. In- is the form derived from Latin, and is therefore used in learned words or in words derived from Latin or (rarely) Greek: inaccessible, inaccuracy, inadequate, etc. Un- is the native form going back to Old English, used in words of native origin, and sometimes used in combination with words of other origins if these words are in common use: unloving, ungodly, unfeeling, unnecessary, unsafe.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for in-

in-1

il-, im- or ir-

prefix
  1. not; non-incredible; insincere; illegal; imperfect; irregular Compare un- 1
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Word Origin

from Latin in-; related to ne-, nōn not

in-2

il-, im- or ir-

prefix
  1. in; into; towards; within; oninfiltrate; immigrate
  2. having an intensive or causative functioninflame; imperil
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Word Origin

from in (prep, adv)
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

prefix meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "not," cognate with Greek an-, Old English un-, from PIE *ne "not" (see un- (1)).

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2

element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in" (see in). In Old French this often became en-, which usually was respelled in English to conform with Latin, but not always, which accounts for pairs like enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (cf. Old English onliehtan "to enlighten"), and some verbs survived into Middle English (cf. inwrite "to inscribe"), but all now seem to be extinct. Not related to in- (1) "not," which also was a common prefix in Latin: to the Romans impressus could mean "pressed" or "unpressed."

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Online Etymology Dictionary, © 2010 Douglas Harper