Bee's wing/beeswing

Originally, this was the name given to the extremely thin, almost transparent, film of fermentation crust that sometimes appears at the top of very old bottles of port or red wine. It dates in this sense from about 1860. Its more recent usage is to describe a very small amount, distance, or thickness of anything. For example, in hanging a picture on a wall, one might want to move it up or down 'a bee's wing'.