English Words in Action, Group X +

The use of the letter x to indicate "unknown quantity" goes back to the French philosopher-mathematician Rene Descartes, who introduced x, y, z, and such usage into the mathematical discourse proceeding from an originally Arabic paradigm (standard or typical example).

By the 18th century, the usage was extended from math to everyday language, and X in the sense of "unknown person" has been in our language ever since.

The term x-rays were used by their discoverer, Wilhelm Roentgen, because their nature and source were unknown.

X-rated language (obscene or offensive language) is heard too often on certain talk-show programs and it interferes with the listener's concentration on whatever viewpoint the vulgar speaker is trying to present.

Primarily a reference to a movie which is considered not suitable to be seen by people under the age of 17 in the U.S. and under 18 in the United Kingdom because of violence, offensive language, or sexual activity.