ラテン語のregula, straightedge, ruleから略され、ラテン語のregere, to straight, to regulateが語源。
英語の語源
rule
rule: [13] Rule is one of a largish family of English words that go back ultimately to Latin rēgula ‘straight stick, ruler, rule, pattern’ (whose close relatives rēx ‘king’ and regere ‘rule’ have also contributed royally to English vocabulary in the form of rector, regent, regiment, royal, etc). Derivatives have produced regular and regulate, while rēgula itself has given rail ‘bar’ and, via Vulgar Latin *regula and Old French reule, rule. => rail, raj, rector, regal, regent, regular, regulate, royal
rule (n.)
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" (see regal). Replaced Old English wealdan.
Meaning "regulation governing play of a game, etc." is from 1690s. Phrase rule of thumb first attested 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.
rule (v.)
c. 1200, "to control, guide, direct," from Old French riuler "impose rule," from Latin regulare (see regulate). Legal sense "establish by decision" is recorded from early 15c. Meaning "mark with lines" is from 1590s. Meaning "to dominate, prevail" is from 1874. "Rule Brittania," patriotic song, is from 1740. Related: Ruled; ruling.