Interesting! I can’t actually say on that one; to me, “spanking” sounds like an old fashioned intensifier I’ve heard “brand spanking new” a few times, which feels like the same kind of use. As to whether that has anything to do with the sail, I’m not sure. It looks like the sail itself was introduced in the late 18th century; in Seamanship in the Age of Sail, John Harland reports that one William Nicholson complains about the new sail design in a book of his in 1792. That’s the closest I can get to origin of the term.