Source : Webster's Revised Unabridged Dictionary (1913)

Sap \Sap\, v. t. [imp. & p. p. {Sapped}; p. pr. & vb. n.
   {Sapping}.] [F. saper (cf. Sp. zapar, It. zapare), fr. sape a
   sort of scythe, LL. sappa a sort of mattock.]
   1. To subvert by digging or wearing away; to mine; to
      undermine; to destroy the foundation of.

            Nor safe their dwellings were, for sapped by floods,
            Their houses fell upon their household gods.
                                                  --Dryden.

   2. (Mil.) To pierce with saps.

   3. To make unstable or infirm; to unsettle; to weaken.

            Ring out the grief that saps the mind. --Tennyson.

Source : WordNet®

sap
     n 1: a watery solution of sugars, salts, and minerals that
          circulates through the vascular system of a plant
     2: a person who lacks good judgment [syn: {fool}, {saphead}, {muggins},
         {tomfool}]
     3: a piece of metal covered by leather with a flexible handle;
        used for hitting people [syn: {blackjack}, {cosh}]
     [also: {sapping}, {sapped}]

sap
     v 1: deplete; "exhaust one's savings"; "We quickly played out our
          strength" [syn: {run down}, {exhaust}, {play out}, {tire}]
     2: excavate the earth beneath
     [also: {sapping}, {sapped}]

sapping
     See {sap}