The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Nor safe their dwellings were, for sapped by floods,
            Their houses fell upon their household gods.
                                                  --Dryden.
      [1913 Webster]

   2. (Mil.) To pierce with saps.
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   3. To make unstable or infirm; to unsettle; to weaken.
      [1913 Webster]

            Ring out the grief that saps the mind. --Tennyson.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "sapped":
   ausgespielt, burned-out, devitalized, disabled, drained, effete,
   enervated, enfeebled, eviscerated, exhausted, fatigued,
   incapacitated, played out, run-down, spent, used up, wasted,
   weakened, worn, worn-out