"cathy,this lamb of yours threatens like a bull!"he said."It is in danger of splitting its skull against my knuckles."

‘Cathy, this lamb of yours threatens like a bull!’ he said. ‘It is in danger of splitting its skull against my knuckles. By God! Mr. Linton, I’m mortally sorry that you are not worth knocking down!’

Heathcliff is threatening Linton, who is questioning Cathy's continuing friendship with a man who demeans and speaks to his wife inappropriately.