1. something difficult or impossible to change
the plan/schedule/rule isn't set in stone
If something is 'set in stone' it is not easy to change or alter. Something considered 'set in stone' is considered to be a firm plan which is impossible or very unlikely to change. This expression is used mostly to talk about plans and is related to the expression 'pencil it in'. If something is written in pencil it is easily changed or erased, something carved or 'set in stone' however is there forever. "The date was pencilled in for the 15th of November and Herbert has approved it so you can consider that as set in stone for the launch date." This is an idiomatic expression used in both social and professional contexts.