c. 1500, "in a serious manner, with serious thought or application," from serious (adj.) + -ly (2). By 1765 as "to a serious extent, so as to give grounds for alarm." To take (something) seriously "be earnest in one's attitude toward" is from 1782.
Middle English also had seriousli "minutely, in detail" (late 14c., now obsolete), from Medieval Latin seriose "item by item, in detail," which reflects the confusion with the stem of series, for which confusion see serious (adj.). Caxton (mid-15c.) uses seriately for "attentively, earnestly."