hackle, n. — Green’s Dictionary of Slang

hackle n.

[SE hackles, the long, shining feathers on the necks of certain birds, typically the domestic cockerel]

courage, pluck; thus show hackle, to be willing to fight.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 152: ‘To show hackle,’ to be willing to fight.
1873		Sl. Dict.