Can any body explain "go moan for man" in JK's On the road?

Context: “I was standing on the hot road underneath an arc-lamp with the summer moths smashing into it when I heard the sound of footsteps from the darkness beyond, and lo, a tall old man with flowing white hair came clomping by with a pack on his back, and when he saw me as he passed, he said, “Go moan for man,” and clomped on back to his dark. Did this mean that I should at last go on my pilgrimmage on foot on the dark roads around America?”

I’m using English as a foreign language and “On the road” is really a big challenge. However, it’s so great that i couldn’t help try to understand as much as possible. Thanks in advance