sap 1 
(săp)
(săp)n.
1.
a. The watery fluid that circulates through a plant, carrying food and other substances to the various tissues.
b. See cell sap.
2. Health and energy; vitality: The constant bickering drained his sap away.
3. Slang A foolish or gullible person.
tr.v. sapped, sap·ping, saps
1. To drain (a tree, for example) of sap.
2. To deplete or weaken gradually: The noisy children sapped all my energy. The flu sapped him of his strength. See Synonyms at deplete.
[Middle English, from Old English sæp. V., sense 2, probably partly from SAP2 (taken as "to weaken (resistance) as by draining of sap.").]