late 14c., etik (in fever etik "hectic fever"), from Old French etique "consumptive," from Late Latin hecticus, from Greek hektikos "continuous, habitual," also used of slow, continued diseases or fevers. The Greek adjective is from hexis "a habit (of mind or body)," from ekhein "have, hold, continue" (from PIE root *segh- "to hold"). The Latin -h- was restored in English 16c.
The use of the word by the Greek physicians apparently was from the notion of a fever rooted in the constitution of the body and symptomatic of one's physical condition, or else from its continuousness (compare ephemera). Hectic fevers are characterized by rapid pulse, flushed cheeks, hot skin, emaciation. In English applied particularly to the wasting fevers, rising and falling with the hours of the day, characteristic of tuberculosis.
Sense of "feverishly exciting, full of disorganized activity" is from 1904 and was a vogue word at first, according to Fowler, but hectic also was used in Middle English as a noun meaning "feverish desire, consuming passion" (early 15c.). Related: Hecticness.