Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Source : WordNet®

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