The term 'x-rated' is a modern compound originating from the American film rating system established in 1968. The 'X' was adopted as a classification symbol, not an abbreviation, chosen for its cultural connotations of mystery, prohibition, and extremity, akin to its use on maps to mark a treasure or a hazard. The second element, 'rated', directly indicates the action of assigning this classification. Thus, the literal construction "classified with an X" evolved into a fixed adjective describing content deemed suitable only for adults, primarily due to sexually explicit material, and later became a broader metaphor for anything extremely indecent or explicit.

Structure: X + rated - X: English. A symbol used to represent an unknown quantity or to mark something as special, prohibited, or extreme. - rated: English. Past participle of the verb 'rate', meaning classified or given a particular rank or evaluation.