means A straight strip of wood, metal, or plastic marked with units, used for measuring lengths and drawing straight lines — or, the person who governs a place.

from From Old French 'reuler' and Latin 'regula,' a straight stick or bar, hence a rule or pattern, all rooted in 'regere,' to keep straight, guide, govern — a cousin of 'regal' and 'regiment.' The same Latin thread ties together the thing that measures lines and the person who lays down the law: both, in the old sense, keep things straight.