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.