The Collaborative International Dictionary of English v.0.48:

Billethead \Bil"let*head`\ (b[i^]l"l[e^]t*h[e^]d), n. (Naut.)
   A round piece of timber at the bow or stern of a whaleboat,
   around which the harpoon line is run out when the whale darts
   off.
   [1913 Webster]