This expression means something that makes you feel aggressively irritated or fearful. 'Hackles' are the erectile hairs along the back of the neck of an animal, especially of a dog. Many animals, particularly dogs and wolves, instinctively raise their hackles when annoyed, threatened or scared. It's presumably primarily done to make the animal look larger and more menacing itself, providing some slight greater degree of protection to the neck area.

Übersetzung:

die Nackenhaare sträuben

Gegenteil(e)

The opposite of "to get one's hackles up" is "to relax".

Synonym(e):

You could also say: to be extremely irritated.