出典:Wiktionary
Unknown. Possibly from X, a graphic derivative of the skull and crossbones (“indicator of death”). The pornography sense may derive from the movie rating of X, which was the rating of movies with mild sexual content in the 1960s and 1970s. The original X certificate was issued from 1951 by the British Board of Film Censors.
XXX
XXX (comparative more XXX, superlative most XXX)
From having three X chromosomes (かつ no Y).