Pip meets someone on the marsh and thinks, “It’s the young man.” What young man? Why does Pip “dare say I should have felt a pain in my liver, too, if I had known where it was?”

Compeyson is the "young man" that Magwitch refers to. Pip is afraid of the young man because Magwitch told him the "young man" would have his heart and liver..... that he was even more fearsome than Magwitch himself.