When someone's hackles rise or go up, they become angry, defensive, or ready to fight. The phrase comes from the way some animals look when threatened: dogs and cats have special hairs on their necks and backs that stand straight up when they're alarmed or aggressive, making them look bigger and more intimidating.

You've probably seen a frightened cat with its fur puffed up, or an angry dog with the fur along its spine bristling. Those standing-up hairs are the hackles. Roosters have hackles too: those long, pointed feathers around their necks that flare out during a fight.

When we say a comment “raised someone's hackles,” we mean it made them instantly defensive or angry. Maybe your friend's hackles rise when someone criticizes their favorite book, or a teacher's hackles go up when students talk during a lesson. The image is perfect: just like those animal hairs standing on end, the person is reacting to a perceived threat or insult, getting ready to defend themselves or push back. It's that sudden feeling of bristling anger or defensiveness, like your whole body is saying “back off!”