A thin straight bar, especially of wood or metal. In former British usage, a rod was a unit of measurement, the equivalent of a perch.
make a rod for one's own back do something likely to cause difficulties for oneself later.
rule with a rod of iron control or govern very strictly or harshly.
spare the rod and spoil the child if children are not physically punished when they do wrong their personal development will suffer. It is proverbial, from the early 11th century, and often with biblical allusion to Proverbs 13:24.