Etymology Quiz

Shakespeare used the word 'X' to refer to a theater and in the 17th century there was theater in London with that name. From Henry V :

Can this 'X' hold The vastie fields of France? Or may we cramme Within this Woodden O, the very Caskes That did affright the Ayre at Agincourt?

In the 18th century, 'X' entered naval jargon, denoting the junior officer quarters on the lowest, or orlop, deck of a ship that doubled for the surgeon’s hospital during battle. Presumably, this was because the midshipmen, like roosters, would get into fights with one another. From William Falconer’s 1769 An Universal Dictionary of the Marine:
'X' of a ship of war, the apartments of the surgeon and his mates, being the place where the wounded men are dressed.
In 1914, the term was applied to airplanes.So the literal sense gave way to a naval metaphor, which was later applied to airplanes. Which word's origin are we talking about?