What does it mean when there is an aggressive yawn from a guinea pig?

When a guinea pig is yawning its a sign of aggression but it's also a sign that a guinea pig is relaxed and happy, so it can be confusing. If the yawning is followed by teeth chattering, it's probably an aggressive yawn, if it's followed by a stretch, it's probably good and it means your guinea pig is happy and relaxed. Even if it is an aggressive yawn, as long as there is no other signs of aggression, he probably is just trying to show the other one who is boss.