Etymology
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rhythm (n.)

 16c. spelling variant or attempted classical correction of Middle English rime "measure, meter, rhythm," also "agreement in end-sounds of words or metrical lines, rhyme; a rhyming poem" (12c.), from Old French rime "verse," from Latin rhythmus "movement in time," from Greek rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement, order; form, shape, wise, manner; soul, disposition," related to rhein "to flow" (from PIE root *sreu- "to flow"). Compare rhyme.

The word rhyme has no connection with the word rhythm, nor is rhyme necessary to accentual verse. Nevertheless, rhyme was usually present. On the other hand, in classical Greek metrical poetry, rhymes, if not accidental, were never an essential element of metrical verse structure. [Henry Osborn Taylor, "The Classical Heritage of the Middle Ages," 1911]

The spelling fluctuated 16c.-17c., rithme and ri'me also being used. From 1550s as "metrical movement, movement in time characterized by equality of measures and alteration of stress and relaxation." By 1776 as "regular succession of beats or accents in music."

The rhythm method in reference to birth control is attested from 1936. Rhythm and blues, U.S. music style, is from 1949 (first in Billboard magazine).

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biorhythm (n.)
also bio-rhythm, "cyclic variation in some bodily function," 1960, from bio- + rhythm. Related: Biorhythmic.
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arrhythmic (adj.)
"without rhythm," 1844 (arhythmic), in relation to musical sensibility, Modern Latin, from Greek arrhythmos "irregular, unrhythmical, without measure," from a- "not" (see a- (3)) + rhythmos "measured flow or movement, rhythm; proportion, symmetry" (see rhythm). Related: Arrhythmically.
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rhythmic (adj.)

c. 1600, "pertaining to rhythm in art," from French rhythmique or directly from Latin rhythmicus, from Greek rhythmikos, from rhythmos "measured flow or movement, rhythm; proportion, symmetry" (see rhythm). By 1630s of language, verse, music, etc., "marked by regularly recurring accents or beats," especially if strong or smooth. Related: Rhythmical (1560s); rhythmically.

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arrhythmia (n.)
in medicine, "irregularity of pulse" (arrhythmia cordis), 1888, from Greek noun of action from arrhythmos "irregular, unrhythmical," from a- "not" (see a- (3)) + rhythmos "measured flow or movement, rhythm; proportion, symmetry" (see rhythm). Nativized form arrhythmy, in reference to metrics, is attested from 1844.
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eurythmic (adj.)

also eurhythmic, "harmonious," 1831, from Greek eurythmia "rhythmical order," from eurythmos "rhythmical, well-proportioned," from eu "well, good" (see eu-) + rhythmos "measured flow or movement, rhythm; proportion, symmetry" (see rhythm). Related: Eurythmics (also eurhythmics), "system of rhythmical body movement to music, used as therapy or to teach musical understanding," developed by Swiss music educator Émile Jaques-Dalcroze; eurythmy.

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rhyme (v.)

a modern spelling variant or replacement of Middle English rime, rimen, from Old French rimer, from rime "verse" (see rhyme (n.)). The Middle English word is attested from late 12c. as "poetic measure, meter," from c. 1300 as "agreement in terminal sounds of words or metrical lines; a rhyming song or ballad."

 The spelling shifted from mid-17c. by influence of rhythm and Latin rhythmus, from the same Greek source, and the intermediate form rhime is frequent for a while (Dryden and Steele have rhime; Pope and Scott rhyme). Related: Rhymed; rhyming; rhymer (Middle English rimer, early 15c., from rime, also from Anglo-French rimour, Old French rimeur).

The poetaster's rhyming dictionary is attested from 1775 (in John Walker's introduction to his "Dictionary of the English Language, Answering at once the Purposes of Rhyming, Spelling, and Pronouncing. On a Plan Not Hitherto Attempted"). The phrase rhyming slang for the Cockney disguised speech in which a word is replaced by a phrase which rhymes with it is attested from 1859 (the thing itself described by 1851). Especially if the rhyming word is then omitted, which seals the reference from the uninitiated: Richard, in rhyming slang "a girl" (a couple of likely Richards), short for Richard the Third, chosen to rhyme with bird "girl."

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magical (adj.)

1550s, "of or pertaining to magic;" c. 1600, "resembling magic in action or effect," from magic (n.) + -al (1). Related: Magically. The difference between magic (adj.) and magical is largely poetic, depending on the rhythm.

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offbeat (adj.)

also off-beat, "unusual," 1938, from off (adv.) + beat (n.). From earlier sense in reference to the second and fourth beats in a four-beat music rhythm (1927), where the stress typically fell on the first and third. Compare upbeat.

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modus (n.)
"way in which anything is done," 1640s, from Latin modus (plural modi) "measure, extent, quantity; proper measure, rhythm, song; a way, manner, fashion, style," from PIE root *med- "take appropriate measures." Especially in modus operandi and modus vivendi.
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