In American legal jargon, lascivious is a semi-technical term indicating immoral sexual thoughts or actions. It is often used in the legal description of criminal acts in which some sort of sexual activity is prohibited to differentiate that activity from "innocent" conduct. It is often used as one of several adjectives to describe pornography as compared to non-pornographic depictions of sex or sexual themes.
In US law consider for example prohibition of mailing lascivious matter:
"Every obscene, lewd, lascivious, indecent, filthy or vile article, matter, thing, device, or substance...[i]s declared to be nonmailable matter and shall not be conveyed in the mails or delivered from any post office or by any letter carrier."
"Lascivious" is not limited to pornography, however. For example,"lascivious cohabitation" refers to a mostly archaic crime of living with a member of the opposite sex, and having sexual intercourse with him or her without first entering a legal or religious marriage.
Similar legal topics include obscenity. ==References==