Old English hacele "cloak, mantle" (cognate with Old High German hachul, Gothic hakuls "cloak;" Old Norse hekla "hooded frock"), of uncertain origin. Sense of "bird plumage" is first recorded early 15c., though this might be from unrelated Middle English hackle "flax comb" (see heckle, n.) on supposed resemblance of comb to ruffled feathers. Metaphoric extension found in raise one's hackles (as a cock does when angry) is first recorded 1881.