late 14c., frenetik, "temporarily deranged, delirious, crazed," from Old French frenetike "mad, crazy" (13c.), from Latin phreneticus "delirious," alteration of Greek phrenitikos, from phrenitis "frenzy," literally "inflammation of the brain," from phren "mind, reason," also "diaphragm" (see phreno-) + -itis "inflammation." The classical ph- sometimes was restored from mid-16c. (see phrenetic). Related: Frenetical; frenetically. Compare frantic.