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-, em-, im-3.[ME, OE; see IN]in-2a prefix of Latin origin meaning primarily "in," but used also as a verb-formative with the same force as in- 1 (incarcerate; incantation).[ < L, comb. form of in (prep.); c. IN]in-3a prefix of Latin origin, corresponding to English un-, having a negative or privative force, freely used as an English formative, esp. 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).[ < L; akin to AN-1, A-6, UN-1]Syn. 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.
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