Etymology
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inflect (v.)

early 15c., "to bend inward," from Latin inflectere (past participle inflexus) "to bend in, bow, curve," figuratively, "to change, alter, influence," from in- "in" (see in- (1)) + flectere "to bend" (see flexible). Grammatical sense "to vary by change of form" (especially at the end of a word) is from 1660s. Related: Inflected; inflecting.

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inflected (adj.)
1640s, "bent, curved," past-participle adjective from inflect (v.). Grammatical sense is from 1775.
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uninflected (adj.)
1713, from un- (1) "not" + past participle of inflect (v.).
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inflection (n.)

also inflexion, early 15c., from Latin inflexionem (nominative inflexio) "a bending, inflection, modification," noun of action from past participle stem of inflectere "to bend in, to change" (see inflect). For spelling, see connection. Grammatical sense "variation by declension or conjugation" is from 1660s; pronunciation sense "modulation of the voice" is from c. 1600.

"Derivation" can be defined as the process by which lexical items belonging to different word-classes are drawn from given bases. Derivation must be distinguished from inflexion, by which different paradigmatic forms are created from given stems. Inflexion describes plural formations, forms of comparison, etc. Inflexion processes do not change the word-class to which the lexical item under consideration belongs. [Alfred Bammesberger, "English Etymology," Heidelberg, Carl Winter, 1984]
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decline (v.)

late 14c., "to turn aside, deviate" (a sense now archaic), also "sink to a lower level," and, figuratively, "fall to an inferior or impaired condition," from Old French decliner "to sink, decline, degenerate, turn aside," from Latin declinare "to lower; avoid, deviate; bend from, inflect," from de "from" (see de-) + clinare "to bend" (from PIE *klein-, suffixed form of root *klei- "to lean").

In grammar, "to inflect as a noun or adjective," from late 14c. The sense has been altered by interpretation of de- as "downward;" intransitive meaning "to bend or slant down" is from c. 1400. Sense of "not to consent, politely refuse or withhold consent to do" is from 1630s. Related: Declined; declining.

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indeclinable (adj.)
late 14c., originally in grammar, from French indéclinable or directly from Latin indeclinabilis "unchangeable," also in grammar, from indeclinatus "unchanged, constant," from in- "not, opposite of" (see in- (1)) + declinatus, from declinare "to lower; avoid, deviate; bend from, inflect" (see decline (v.)). Related: Indeclinably.
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conjugate (v.)

1520s, in the grammatical sense, "inflect (a verb) through all its various forms," from Latin coniugatus, past participle of coniugare "to yoke together," from assimilated form of com "with, together" (see con-) + iugare "to join," from iugum "yoke" (from PIE root *yeug- "to join"). "This use has its origin in the fact that in inflected languages, a verb is conjugated by conjoining certain inflectional syllables with the root" [Century Dictionary]. Earlier as an adjective, "joined together" (late 15c.). Related: Conjugated; conjugating.

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modulate (v.)

1610s, in music, "vary or inflect the sound of," especially to give expressiveness, "vary the pitch of," back-formation from modulation, or else from Latin modulatus, past participle of modulari "regulate, measure off properly, measure rhythmically; play, play upon," from modulus "small measure," diminutive of modus "measure, manner" (from PIE root *med- "take appropriate measures").

General sense of "modify, adjust, adapt, regulate in measure or proportion" is from 1620s. The intransitive musical sense of "pass from one key to another, or between major and minor" is attested by 1721. In telecommunications from 1908. Meaning "exert a controlling influence on, regulate" is by 1964. Related: Modulated; modulating.

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declension (n.)

mid-15c., declinson, in grammar, "the inflection of nouns, pronouns, and adjectives, especially with a change in form from the nominative case," ultimately from Latin declinationem (nominative declinatio) "a bending from (something), a bending aside; a turning away from (something), an avoiding," also used in the grammatical sense, noun of action from past-participle stem of declinare "to lower; avoid, deviate; bend from, inflect," from de "from" (see de-) + clinare "to bend" (from PIE *klein-, suffixed form of root *klei- "to lean").

The immediate source of the English word is perhaps in French (compare Old French declinaison), but "the form is irregular, and its history obscure" [OED]. Meaning "a sloping downward" is from 1640s; that of "a sinking or falling into a lower or inferior state" is from c. 1600; that of "courteous refusal, non-acceptance" is by 1817. Related: Declensional.

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