Where does the term “throttle quadrant” come from?
The term quadrant refers to the panel containing the levers and is derived the circular arc in which the throttles are placed:


  throttle quadrant
  
  The recess in which the throttle is located and moves. ... It derives its name from the circular arc in which the throttle is placed. 


Looks like the term entered aviation from the rudder controls in a ship:


  quadrant
  
  
  A frame fixed to the head of a ship's rudder, to which the steering mechanism is attached.
  
  3.1 A panel with slots through which a lever is moved to orient or otherwise control a mechanism.
  


Here's a rudder quadrant



Rudder quadrant; image from titanic-model.com