Professional pilot slang for the flight simulator they must periodically visit (usually every six or nine months), wherein they thrash, sweat and strain with the aircraft equipment malfunctions, foul weather and Air Traffic Controller screw-ups that their sadistic Sim Instructor (chuckling "heh, heh") loads them up with from his control panel behind the pilots in the simulator. The box can be a real E-ticket ride. Mounted on hydraulic struts/actuators that can produce pitch, roll and yaw as well as air turbulence (even the feel of the aircraft taxiing on pavement); the actuators are a treat to watch in action from outside the simulator as they tilt the boxy structure this way and that - much more pleasant than experiencing the motion from the inside!
Obviously, simulators save airlines money and are invaluable for practicing emergency procedures (see red box items) and inculcating habit patterns that will serve a pilot in a real-world actual emergency.
Still, most pilots look forward to their time in "the box" with as much enthusiasm as they do going to the dentist for a root-canal!