Origin and history of iambic

in prosody, "pertaining to or consisting of choriambs," 1650s, from Latin choriambicus, from Greek khōriambikos, from khōriambos. This is compounded from iambos (see iambic) + khoreios, the name of the foot we tend to call a trochee, literally "pertaining to a dance or theatrical chorus," from khoros (see chorus).

In classical prosody a four-syllable foot, the first and last long the middle two short. Common in English poetry 16c.-19c. (“Lilies without, roses within”), but in English it is less a foot than a two-foot pattern of an inverted iamb (a trochee, or choreus) followed by an iamb, typically at the start of an iambic decasyllable line or after a caesura. As a noun, "a foot constituting a choriamb," by 1866.

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 English as in Greek, it has been held to be the natural cadence of speech. The full Greek iamb consisted of two such units, one variable the other weighted like a modern English iamb. 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]

Compare trochee, spondee. The Greeks gave names to recurring patterns imparted to the words of their ritual songs and dances. The patterns were associated with certain types of songs and dances, and tended to take their names accordingly. The Roman poets picked up the vocabulary from the Greeks and applied it, somewhat ill-fitted, to their own (undanced) verses.

The English poets of the 16c., building a prosody for modern English, hesitated but then accepted the Latin foot names and applied them to stress patterns in English that, in only some ways, approximate those of Latin. Consequently the Greek meanings of the foot-names have almost no relevance to the modern use of them in prosody.