Английский Этимологический словарь - fit (2)

- "sudden attack" (as of anger), probably from O.E. fitt "conflict, struggle," of uncertain origin. Meaning "a sudden sharp attack, paroxysm" is from 1547. Fitful was used once by Shakespeare ("Macbeth" iii.2) in sense of "characterized by fits," then revived c.1810 with a sense of "shifting, changing."