seriously adv.

early 15c., "arranged in sequence, continuous" (a sense now obsolete); mid-15c., of persons, "expressing earnest purpose or thought, resolute," from Old French serios "grave, earnest" (14c., Modern French sérieux) and directly from Late Latin seriosus, from Latin serius "weighty, important, grave."

This is probably from a PIE root *sehro- "slow, heavy" (source also of Lithuanian sveriu, sverti "to weigh, lift," svarus "heavy, weighty;" Old English swær "heavy," German schwer "heavy," Gothic swers "honored, esteemed," literally "weighty").

According to Middle English Compendium, two sets of Latin stems "seem to have fallen together" in Medieval Latin: ser- (as in series, serere) and sēr- (as in sērius, sēriōsus, etc.), perhaps through semantic overlap, which accounts for the earlier Middle English record of the word, which seems to belong to the first stem.

As "in earnest, not pretending or jesting," from 1712; in reference to of music, theater, etc., "dealing with grave matters" by 1762. The meaning "attended with danger, giving grounds for alarm" is from 1800. Serious-minded is attested by 1845.

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.

1610s, "a number or set of things of one kind arranged in a line, a continued succession of similar things," also of events following in order, from Latin series "row, chain, series, sequence, succession," from serere "to join, link, bind together, arrange, attach, put; join in speech, discuss" (from PIE root *ser- (2) "to line up"). The earlier word was serie "sequence of thoughts, discussion of a subject" (late 14c.).

The meaning "set of printed works published consecutively and having something in common" is from 1711. The meaning "set of radio or television programs with the same characters and themes" is attested from 1949. The baseball sense of "set of games on consecutive days between the same teams" is from 1862.