Hackles are the hairs on an animal's neck that rise in anger. Raise his hackles, hackles up, the dog's hackles rose.
The figurative use is the commoner, and raising somebody's hackles means annoying them. In a dog the raised fur is involuntary and signals arousal rather than aggression specifically, which is worth knowing before reading it as a threat.
1nounhacklesThe hairs on an animal's neck that rise in anger.
“The dog's hackles rose, and it growled at the stranger by the gate.”
hackles is a hair.