The Collaborative International Dictionary of English v.0.48:

Treenail \Tree"nail`\, n. [Tree + nail.] (Shipbuilding)
   A long wooden pin used in fastening the planks of a vessel to
   the timbers or to each other. [Written also trenail, and
   trunnel.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

treenail
    n 1: a wooden peg that is used to fasten timbers in
         shipbuilding; water causes the peg to swell and hold the
         timbers fast [syn: treenail, trenail, trunnel]