Short form of specification — a statement of exactly what a thing must be. A spec is a contract before it is a plan.
Latin species (a kind, an appearance) + facere (to make), by way of specification; a statement of exactly what a thing must be, a contract before it is ever a plan.
A statement agreed before the building starts, so that disagreement about what a thing should be happens on paper rather than after the effort of making it is already spent.