When a cat hisses, it means: fear, warning, pain, seeking help, or illness.
1. Fear
When cats go to a new environment or encounter new things for the first time, they will feel scared and fearful. For example, sometimes cats that are usually very docile at home will hiss at strangers when they go out. They are trying to embolden themselves by hissing because they think they are scared. 
2. Warning
If a cat senses that a person or animal may harm it, it will hiss to warn you, telling you that it is not to be trifled with, and also putting itself into a fighting state. If you attack at this time, it will immediately retaliate.
3. Pain
Some cats may not be able to express themselves well, but they are also more tolerant of pain. When they feel uncomfortable, they will pant constantly in hopes of relieving the pain.
4. Seeking help
Sometimes when a cat hisses at a person, it is actually asking for help, especially when it feels hungry or tired and its body has no energy reserves, it is easy for it to hiss.
5. Illness
Respiratory infections in cats can cause airway obstruction, leading to persistent panting. Symptoms of feline respiratory infections mainly include sneezing, runny nose, nasal congestion, and coughing. If you notice these symptoms in your cat, seek immediate medical attention.
