If I had thee in Lipsbury pinfold, I would make thee care for me:
At the opening of the scene Oswald takes Kent for just another servant, not recognizing him as the man who punished him when he snubbed King Lear. (See 1.4.77-92.)
In the present scene Kent has just told Oswald "I love thee not," and Oswald has replied, "Why, then, I care not for thee," meaning, "If you are going to be that way, I don't like you either." Kent's answer "If I had thee in Lipsbury pinfold, I would make thee care for me" means "If I had you between my teeth I would make you care about who I am."
A pinfold is a pound for stray animals (usually a round stone enclosure); "Lipsbury" is Kent's invented place-name.
Pinfold Pound with Cone Source:
Geog Port UK