sap1
  
  n.
  - the fluid, chiefly water with dissolved sugars and mineral salts, circulating in the vascular system of a plant.
 - vigour or energy.
 
v.
 (saps, sapping, sapped)
 gradually weaken (a person's strength or power). 
▸
(sap someone of)
 drain someone of (strength or power).
 
  Derivative
  
  Etymology
  OE sæp, prob. of Gmc origin; the verb is derived orig. from the verb sap2, in the sense ‘undermine’.
 
  
    
sap2
  
  n.
 historical a tunnel or trench to conceal an assailant's approach to a fortified place.
v.
 (saps, sapping, sapped)
   - historical dig a sap.
 - archaic make insecure by removing the foundations.
 - 
[often as noun sapping]
 Geography  undercut by water or glacial action.
 
 
  Etymology
  C16: from Fr. saper, from Ital. zappare, from zappa ‘spade, spadework’, prob. from Arab. sarab ‘underground passage’, or sabora ‘probe a wound, explore’.
 
  
    
sap3
  
  n.
 informal, chiefly N. Amer. a foolish person.
 
  Etymology
  C19: abbrev. of dial. sapskull ‘person with a head like sapwood’.