A bounded place where things may be tried without consequence outside it. Safety made by walls rather than by care.
Old English sand joined to box (ultimately from Latin buxus, boxwood); the child's sandpit lent its name to computing's isolated testing space in the later twentieth century, keeping the same bounded-place-to-try-things idea.
A bounded place for trying things where nothing that happens there reaches outside it — safety built from walls rather than earned through care.