c. 1200, "principle or maxim governing conduct, formula to which conduct must be conformed" from Old French riule, Norman reule "rule, custom, (religious) order" (in Modern French partially re-Latinized as règle), from Vulgar Latin *regula, from Latin regula "straight stick, bar, ruler;" figuratively "a pattern, a model," related to regere "to rule, straighten, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). Replaced Old English wealdan.
Meaning "regulation governing play of a game, etc." is from 1690s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Meaning "strip used for making straight lines or measuring" is recorded from mid-14c. Typography sense is attested from 1680s.