late 14c., frenetik, "temporarily deranged, delirious, crazed," from Old French frenetike "mad, crazy" (13c.), from Latin phreneticus "delirious," alteration of Greek phrenitikos, from phrenitis (nosos) "frenzy, mental disease, insanity," literally "inflammation of the brain," from phrēn "mind, reason," also "diaphragm" (see phreno-) + -itis "inflammation." The classical ph- sometimes was restored from mid-16c. (see phrenetic). Related: Frenetical; frenetically. Compare frantic.
mid-14c., "insane," unexplained variant of Middle English frentik (see frenetic). Compare franzy, dialectal form of frenzy. Transferred meaning "affected by wild excitement" is from late 15c. Of the adverbial forms, frantically (1749) is later than franticly (1540s).
<a href="https://www.etymonline.com/word/phrenetic">Etymology of phrenetic by etymonline</a>
Harper, D. (n.d.). Etymology of phrenetic. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/phrenetic