ceremonious adj.

late 14c., cerymonye, "a religious observance, a solemn rite," from Old French ceremonie and directly from Medieval Latin ceremonia, from Latin caerimonia "holiness, sacredness; awe; reverent rite, sacred ceremony," an obscure word, possibly of Etruscan origin, or a reference to the ancient rites performed by the Etruscan pontiffs at Caere, near Rome.

Introduced in English by Wycliffe. Also from late 14c. as "a conventional usage of politeness, formality." The disparaging sense of "mere formality" is by 1550s.

1580s, from un- (1) "not" + ceremonious. Related: Unceremoniously.