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]