Like Freshmen 15, you supposedly will gain 15lbs in your freshmen year. Cubicle 15 is the same, the phenomenon of gaining 15lbs in your first year working in a cubicle.
A: How's your new job?
B: Damn dude, all I do is sit. Cubicle 15, man.
A: I'm getting fat ever since I start that 9-5 life.
B: ikr, #cubicle15.