- The hairs on the back of a dog or cat's neck that rise when the animal is agitated or frightened; figuratively, it can also refer to a state of emotional tension or unease.
When the dog saw the stranger approaching its territory, it bared its teeth and raised the hair along its back, signaling its readiness to defend itself.
Start tacking to remember the definition of "hackles" the next time you hear it or read it.
Yakety Yak