Etymology
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iamb (n.)

in prosody, a foot of two syllables, the first short or unaccented, the second long or accented, 1842, from French iambe (16c.) or directly from Latin iambus "an iambic foot; an iambic poem," from Greek iambos "metrical foot of one unaccented followed by one accented syllable" (see iambic). Iambus itself was used in English in this sense from 1580s. In Greek, the measure was said to have been first used by satiric writers.

[The Iambus] is formed constantly by the proper accentuation of familiar, but dignified, conversational language, either in Greek or English : it is the dramatic metre in both, and in English, the Epic also. When the softened or passionate syllables of Italian replace the Latin resoluteness, it enters the measure of Dante, with a peculiar quietness and lightness of accent which distinguish it, there, wholly from the Greek and English Iambus. [Ruskin, "Elements of English Prosody, for use in St. George's Schools," 1880]
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iambic 

in prosody, 1570s (n.) "a foot of two syllables, the first short or unaccented, the second long or accented;" 1580s (adj.), "pertaining to or employing iambs," from Late Latin iambicus, from Greek iambikos, from iambos "metrical foot of one unaccented followed by one accented syllable; an iambic verse or poem," traditionally said to be from iaptein "to assail, attack" (in words), literally "to put forth, send forth" (in reference to missiles, etc.), but Beekes says "doubtless of Pre-Greek origin."

The meter of invective and lampoon in classical Greek since it was first used 7c. B.C.E. by Archilochus, whose tomb, Gaetulicus says, is haunted by wasps; iambics of various length formed the bulk of all English poetry before 20c. and a great deal since. The iambic of classical Greek and Latin poetry was quantitative.

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Iapetus 

a titan, son of Uranus and Gaia; Latinized form of Greek Iapetos, which is of uncertain origin. It has been connected with biblical Japheth or with Greek iaptein "to put forth, send forth" (perhaps as "the one thrown" by Zeus into Tartaros" or "the one who throws" a spear, etc.). Beekes finds these improbable and suggests that, as the name of a pre-Olympian god, it is a Pre-Greek word. 

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iatrogenic (adj.)
"induced by a physician," 1920, from iatro- + -genic.
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I-beam (n.)
1869; see beam (n.). So called for its shape. I-bar is from 1890; also I-rail (1873).
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ibex (n.)
"chamois, wild goat of the Alps and Apennines," c. 1600, from Latin ibex, which probably is from a pre-Latin Alpine language. The German Steinbock.
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ibid. (adv.)
"at the place or in the book already mentioned" (used to avoid repetition of references), 1660s, abbreviation of Latin ibidem "in the same place, just there," from ibi "there," pronominal adverb of place, + demonstrative suffix -dem. Also ibid, but properly with the period.
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ibis (n.)
stork-like bird, late 14c., from Latin ibis (plural ibes), from Greek ibis, from Egyptian hab, a sacred bird of Egypt.
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ibogaine (n.)
nerve stimulant, 1901, from French ibogaine, from iboga, Congolese name of the shrub from which the chemical is extracted, + chemical suffix -ine (2).
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ICBM (n.)
also I.C.B.M., 1955, initialism (acronym) for Inter-Continental Ballistic Missile. IBM in the same sense is from 1954.
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