late 14c., etik (in fever etik), from Old French etique "consumptive," from Late Latin hecticus, from Greek hektikos "continuous, habitual, consumptive" (of a disease, because of the constant fever), from hexis "a habit (of mind or body)," from ekhein "have, hold, continue" (see scheme).
The Latin -h- was restored in English 16c. Sense of "feverishly exciting, full of disorganized activity" first recorded 1904, but hectic also was used in Middle English as a noun meaning "feverish desire, consuming passion" (early 15c.). Hectic fevers are characterized by rapid pulse, among other symptoms. Related: Hecticness.