What does it mean?
In algebra you replace x with y. Here X means an ex, and Y sounds like "why," so it asks someone to take their ex's place without questions.
How to use it
Good for a math nerd or a classmate, and a fun text if the other person gets algebra jokes. Say it lightly so it doesn't sound like baggage.
How to reply
Solving for X is easy. Explaining your X is the hard part.