Dictionary meaning- the erectile hair on the back of an animals neck
For an easier definition, its the hair that raises along the spine of an animal (in this case a dog) to show its feelings. The most common reason for the hackles to rise is aggression, that’s when you see it the most in dogs. Though, the hackle can also raise when a dog is frightened, extremely nervous, surprised, or even excited.
It’s similar to the way me and you get goosebumps, the dogs way is just a bit more noticeable.
A lot of people label a dogs hackle rising only as a sign of aggression, though, just by paying attention to the rest of the dogs body and posture you can tell for sure how the dog is feeling.
Most likely, if the dog is getting aggressive the hackles will not only raise but the dog’s tail will go up as well as its ears. In severe cases the dog will show its teeth and bark/growl.
If the dog is doing the opposite, lowering to the ground, its tail tucked, and ears flat against its head, its probably nervous or afraid.
So, its important to keep an eye on your dog and notice these things and then take care of the situation before things escalate.