Why is it called "STBY RUD" on a Boeing 737?
According to the Boeing 737 Flight Crew Operation Manual, the "Standby Hydraulic System" only powers the following:


thrust reversers
rudder
leading edge flaps and slats (extend only)
standby yaw damper


It does not power, for example, brakes, ailerons, spoilers and the landing gear.

I speculate that the word is chosen to remind pilots that the system powers the rudder but not other primary flight controls.