|(the) X factor
||Unknown and/or unforeseeable circumstances
|X marks the spot
||Used in locating something
|for XYZ reasons
||For reasons unknown and not worth speculating on.
||Examine your zipper. This expression is a slang rough acronym (initialism). The X stands for the sound of the "ex" in examine resembling the phonetic sound for the letter X in the English alphabet. The other components, the Y and Z are accurate as Y is the first letter of "your" and Z is the first letter of "zipper." (This refers to one finding embarrasment that they have forgetten to zip up their trousers.)