well think of it like this a sketch drawing is not a very clean drawing, When used as slang, the word sketch means that it is not set up really sturdy or well, just like a drawing that is a sketch is done quick and will not be a good as a final drawing. So if something is sketchy it means that it will either probably break bucuase it is not very sturdy, or that it is just what you basicaly need and not finished up.
