regularly adv.

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."

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.