Kassa was horrible for it. It hurts like heck. I said ""ouch" in an exaggerated hurt voice and she stopped. I don't think they know how hard they are nipping so like the Mother dog or pups who yelp I did the same. If she kept it up she was often over stimulated and needed time out.

Both Erns and Tessa were older when I adopted them and both nipped. Tessa with excitment, but Erns could be serious. His owners said he always nipped...yes because you didn't stop him. I did the same when they nipped me and stopped quickly.

One thing Erns owners did do was sit on him, and force him down. With play, or to control him. As he got older he resisted. When they tried to cut his claws or flea treat him 3 adults held him down and he went over the top.
By the time I got him if anyone so much as sat on the floor he would grab them by the neck, rip their clothes, and go into a frenzy. I would often be bitten and if I hadn't seen the good side to him and known he was part of Kassa so couldn't be aggressive he would not be here now. He grabbed a vet who knelt on the floor and a trainer.
I had no idea what it was about until months later someone told me they used to hold him down. Took me almost a year to get him to trust people.
Six years on we can sit on the floor, but have to be very careful something doesn't trigger the fear he had as a pup.