The Collaborative International Dictionary of English v.0.48:

Sustain \Sus*tain"\, v. t. [imp. & p. p. Sustained; p. pr. &
   vb. n. Sustaining.] [OE. sustenen, susteinen, OF. sustenir,
   sostenir, F. soutenir (the French prefix is properly fr. L.
   subtus below, fr. sub under), L. sustinere; pref. sus- (see
   Sub-) + tenere to hold. See Tenable, and cf.
   Sustenance.]
   1. To keep from falling; to bear; to uphold; to support; as,
      a foundation sustains the superstructure; a beast sustains
      a load; a rope sustains a weight.
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            Every pillar the temple to sustain.   --Chaucer.
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   2. Hence, to keep from sinking, as in despondence, or the
      like; to support.
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            No comfortable expectations of another life to
            sustain him under the evils in this world.
                                                  --Tillotson.
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   3. To maintain; to keep alive; to support; to subsist; to
      nourish; as, provisions to sustain an army.
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   4. To aid, comfort, or relieve; to vindicate. --Shak.
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            His sons, who seek the tyrant to sustain. --Dryden.
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   5. To endure without failing or yielding; to bear up under;
      as, to sustain defeat and disappointment.
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   6. To suffer; to bear; to undergo.
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            Shall Turnus, then, such endless toil sustain?
                                                  --Dryden.
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            You shall sustain more new disgraces. --Shak.
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   7. To allow the prosecution of; to admit as valid; to
      sanction; to continue; not to dismiss or abate; as, the
      court sustained the action or suit.
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   8. To prove; to establish by evidence; to corroborate or
      confirm; to be conclusive of; as, to sustain a charge, an
      accusation, or a proposition.
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   Syn: To support; uphold; subsist; assist; relieve; suffer;
        undergo.
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Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "sustaining":
   aid, backing, bearing, bolstering, bracing, burdened, buttressing,
   carriage, carrying, holding, maintaining, maintenance,
   moral support, propping, psychological support, reliance,
   security blanket, shoring, subsidy, subvention, support,
   supporting, supportive, supportive relationship,
   supportive therapy, suspensory, sustainment, sustenance,
   sustentation, sustentative, upholding, upkeep