Upper bounds, and the surfaces overhead. Both are what you cannot pass without breaking something.
Middle English celynge, of debated formation: perhaps from Old French celer (to conceal, to hide) via a verb ceil, or influenced by Latin caelum (sky, ancestor of French ciel); either root suits an overhead surface that hides the room above and stands in for a sky.
Upper bounds, and also the literal surfaces overhead — both name what cannot be passed without breaking something.