"cloak, mantle" (cf. O.H.G. hachul,
"cloak;" O.N. hekla
"hooded frock"). Sense of "bird plumage" is first recorded 1496, though this may be from unrelated M.E. hackle
"flax comb" (see heckle
) 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.