Deciding the shape of a thing before it is made. Design is where most of a thing's fate is settled.
Latin designare (to mark out, designate), from de- (thoroughly) + signare (to mark), from signum (a sign, mark); deciding the shape of a thing before it is made, marking out its plan in advance of the making.
Latin signum is a mark, and design was first the marking-out of a plan, a line drawn before any material touched it. Most of what a made thing becomes was already decided in that mark, long before the making itself began.