What is the X in X-mas?
late 14c., from Old French reguler "ecclesiastical" (Modern French r*#233;gulier), from Late Latin regularis "containing rules for guidance," from Latin regula "rule," from PIE *reg- "move in a straight line" (see regal).
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."
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.
A cup of coffee with the usual moderate amount of cream and sugar •In New York City no sugar is included (1950s+ fr lunch counter)