etymo

Kindly excuse if this has been asked before:

It is widely believed the term X is derived from the 1970 X sketch of the BBC television comedy series "Monty Python's Flying Circus".The sketch is set in a cafe where nearly every item on the menu includes spiced pork and meat. As the waiter recites the menu, a chorus of Viking patrons drowns out all conversations with a song repeating "X, X, lovely X, wonderful X". The excessive amount of X mentioned in the sketch is a reference to British rationing during World War II. X was one of the few meat products that was excluded from rationing, and hence was widely available.

Clue: X is an acronym whose full form is there in the question.