What physical feature did the first man have that pip ran into on the marsh

The man had a "great iron" on his leg. This would mean his leg was shackled.

A fearful man, all in coarse gray, with a great iron on his leg. A man with no hat, and with broken shoes, and with an old rag tied round his head. A man who had been soaked in water, and smothered in mud, and lamed by stones, and cut by flints, and stung by nettles, and torn by briars; who limped, and shivered, and glared, and growled; and whose teeth chattered in his head as he seized me by the chin.