Originating from puppies tendency to return to abusive owners, "puppy-back" is when a person exhibits a pattern of returning/forgiving bad relationships even when fully aware it is bad for them.
John's girlfriend keeps on being a bitch to him, and using him. He knows it, they've ended before. Regardless he never learns to not puppy-back.
Damnit Aly, stop fucking puppy-backing to him. He cheated on you 4 times.