A cur is an unfriendly or aggressive dog, especially a mongrel or mixed-breed dog that's not well-trained or well-behaved. In older times, people used the word to describe stray dogs that might growl, snap, or cause trouble in neighborhoods.

The word carries a harsh, dismissive tone. When someone calls a dog a cur, they're describing it as mixed-breed and suggesting it has poor manners or a mean temperament. It's quite different from calling a dog a “mutt” or “mixed-breed,” which can be said affectionately.

People also use cur as an insult for humans, calling someone a cur when they act in a cowardly, mean-spirited, or dishonorable way. In classic adventure stories and historical novels, you might read about a villain being called a cur after betraying someone or acting without courage or honor. The insult compares the person to that aggressive, untrustworthy dog.

The word appears often in older literature but sounds quite old-fashioned today. You're more likely to encounter it in books set in past centuries than in modern conversation.