sag1
  
  v.
 (sags, sagging, sagged)
 sink, subside, or bulge downwards gradually under weight or pressure or through lack of strength. 
▸hang down loosely or unevenly.
n.
  - an instance of sagging.
 - Geometry  the amount of a downward curve, measured as the perpendicular distance from the middle of the curve to the straight line between the two supporting points.
 
 
  Phrase verbal
  
    
      - sag off
 informal play truant from school. 
    
   
  Derivative
  
  Etymology
  ME: appar. rel. to Mid. Low Ger. sacken, Du. zakken ‘subside’.