If that mean-looking doggie has his back arched, teeth bared and the hairs on the back of his neck are standing up straight, walk away slowly. You've raised his hackles, or instilled a feeling of anger.
In the technical sense, hackles are the hairs or feathers that grow right at the back of an animal's neck. You know, the things that spike up on a scared cat or angry rooster. But in the human world, raising someone's hackles means you've made them boiling mad or super annoyed. For example, spitting spit balls while the teacher isn't looking is sure to raise her hackles. And land you in detention.
