c. 1400, reguler, "belonging to or subject to a religious or monastic rule," from Old French reguler "ecclesiastical" (Modern French régulier) and directly from Late Latin regularis "containing rules for guidance," from Latin regula "rule, straight piece of wood" (from PIE root *reg- "move in a straight line"). The classical -a- was restored 16c.
In earliest use, the opposite of secular. Extended from late 16c. to shapes, verbs, etc., that followed predictable, proper, or uniform patterns. From 1590s as "marked or distinguished by steadiness or uniformity in action or practice;" hence, of persons, "pursuing a definite course, observing a universal principle in action or conduct" (c. 1600).
The sense of "normal, conformed or conforming to established customs" is from 1630s. The meaning "orderly, well-behaved" is from 1705. By 1756 as "recurring at repeated or fixed times," especially at short, uniform intervals. The military sense of "properly and permanently organized, constituting part of a standing army" is by 1706. The colloquial meaning "real, genuine, thorough" is from 1821.
Old English borrowed Latin regula and nativized it as regol "rule, regulation, canon, law, standard, pattern;" hence regolsticca "ruler" (instrument); regollic (adj.) "canonical, regular."
c. 1400, reguler, "member of a religious order bound by vows," from regular (adj.) and from Medieval Latin regularis "member of a religious or monastic order." Sense of "soldier of a standing army" is from 1756. Meaning "regular customer" is by 1852; meaning "leaded gasoline" is by 1978; regular (adj.) in the sense of "unleaded" is by 1974.