lasciviousness (n.)

mid-15c., "lustful, inclined to lust," from Medieval Latin lasciviosus (used in a scolding sense by Isidore and other early Church writers), from Latin lascivia "lewdness, playfulness, fun, frolicsomeness, jolity," from lascivus "lewd, playful, undesigned, frolicsome, wanton."

This is from PIE *las-ko-, from the root *las- "to be eager, wanton, or unruly" (source also of Sanskrit -lasati "yearns," lasati "plays, frolics," Hittite ilaliya- "to desire, covet," Greek laste "harlot," Old Church Slavonic laska "flattery," Slovak laska "love," Russian lasyj "greedy, eager, affectionate," Old Irish lainn "greedy, eager," Gothic lustus, Old English lust "lust").

Meaning "tending to excite lust" is from 1580s. Related: Lasciviously. In 17c. also with a verbal form, lasciviate, now obsolete.

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).