The OED entry for sny as a noun doesn't have a definition, but here are a couple of extracts listed:
1846 A. Young Naut. Dict. 288 In shipbuilding, a plank is said to have sny, when its edge has an upward curve.
1875 E. H. Knight Pract. Dict. Mech. 2232/1 Sny,..the trend of the lines of a ship upward from amidship toward the bow and the stern.