1550s, "rhymed verse, metrical movement," 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" (see rheum). Rhythm method of birth control attested from 1936. Rhythm and blues, U.S. music style, is from 1949 (first in "Billboard").