seriousness (n.)

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.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).