What did the old man have on the back of his neck in Old Man and the Sea?
The old man has "deep wrinkles in the back of his neck".
This detail is given to us by the author in the second paragraph of the book. In that paragraph, the reader receives a physical description of Santiago, who is
"...thin and gaunt with deep wrinkles in the back of his neck. The brown blotches of the benevolent skin cancer the sun brings from its reflection on the tropic sea (are) on his cheeks. The blotches (run) well down the sides of his face and his hands (have) the deep-creased scars from handling heavy fish on the cords...these scars...(are) as old as erosions in a fishless desert".
Through this description, the author emphasizes two important characteristics about Santiago. First of all, there is age - he is old, wrinkled and weathered, and has been a fisherman for a long time. Secondly, there is his profession, which has taken a toll on his body - he has a form of skin cancer from constantly being in the sun, the rigors of his job have left him "thin and gaunt", and his hands are deeply scarred.
The author ends his description with a comment about the old man's spirit, giving the reader a complete picture of Santiago, body and soul. The author says,
"Everything about him (is) old except his eyes and they (are) the same color as the sea and (are) cheerful and undefeated".