Very doubtful - although possibly from X, a graphic derivative of the skull and crossbones (“indicator of death”); more likely taken 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.
- Traditional brewer's mark for barrels of "extra strong" beer - extended to other alcoholic beverages and other products having an "extra strong" dimension (varying from washing soap to pornography).
- An indicator of hazard or poison.
- Extra strong; very high quality
- Designating of hardcore pornography.
- 2012 April 19, Josh Halliday, “Free speech haven or lawless cesspool – can the internet be civilised?”, in the Guardian:
- Other global taboos, such as sex and suicide, manifest themselves widely online, with websites offering suicide guides and Hot XXX Action seconds away at the click of a button. The UK government will come under pressure to block access to pornographic websites this year when a committee of MPs publishes its report on protecting children online.
From having three X chromosomes (and no Y).