means The fluid that circulates through a plant carrying water and nutrients; figuratively, the vital energy of something — or, as a verb, to drain that energy away.

from From Old English 'sæp,' the juice of a plant, with relatives across the Germanic languages (Old High German 'saf,' modern German 'Saft' for juice). The 'sap' meaning a foolish or gullible person is likely a separate word, possibly clipped from 'sapskull.' And the military 'sap' — a covered trench dug to undermine a fortress — comes via French and Italian 'zappa,' a spade or hoe; from that digging-out sense grew the verb 'to sap,' to weaken something gradually from beneath, which is why draining a tree and draining a person's strength now share a word.