If a piece of work was written or planned on the back of an envelope, it was prepared too quickly, with little thought or care. These proposals were sketched out on the back of an envelope. The screenplay sounds as if it's been written on the back of an envelope. Note: You can also talk about a back-of-an-envelope calculation or account to talk about something that is not accurate. According to his back-of-an-envelope calculation, a solar square of mirrors of this size would provide enough energy to supply the whole state.