regular (n.) Look up regular at Dictionary.com
c. 1400, "member of a religious order," from regular (adj.). Sense of "soldier of a standing army" is from 1756. Meaning "regular customer" is from 1852; meaning "leaded gasoline" is from 1978.
regular (adj.) Look up regular at Dictionary.com
late 14c., from Old French reguler "ecclesiastical" (Modern French régulier), 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."

Earliest sense was of religious orders (the opposite of secular). Extended from late 16c. to shapes, etc., that followed predictable or uniform patterns; sense of "normal" is from 1630s; meaning "real, genuine" 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."