The technical term to slop is Backlash.
Slop is the freedom of play in the levers and control systems of a model helicopter which results in it being difficult or unpredictable to fly.
Slop can come from loose ball links, worn servo gear trains or loose output shafts, damaged bearings, or looseness or lack of rigidity of any supporting part of the airframe.
References
Slop (remote control) Wikipedia(Text) CC BY-SA