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.
1560s, "pertaining to or consisting of a rite or rites," from French ritual or directly from Latin ritualis "relating to (religious) rites," from ritus "religious observance or ceremony, custom, usage," (see rite). By 1630s as "done as or in the manner of a rite" (as in ritual murder, attested by 1896). Related: Ritually.