This term originally referred to builders who used their thumb to estimate measurements. The meaning broadened to mean any inexact but helpful rule. "Be specific" is a rule of thumb for most types of writing. "Look both ways before crossing the street" is a rule of thumb for pedestrians. If something is an official or written rule or law, it isn't a rule of thumb. Rules of thumb are unwritten but helpful guidelines. Everything, from parenting to architecture, has its rules of thumb.