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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The Collaborative International Dictionary of English v.0.48:

Sustain \Sus*tain"\, n.
   One who, or that which, upholds or sustains; a sustainer.
   [Obs.]
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         I waked again, for my sustain was the Lord. --Milton.
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WordNet (r) 3.0 (2006):

sustain
    v 1: lengthen or extend in duration or space; "We sustained the
         diplomatic negotiations as long as possible"; "prolong the
         treatment of the patient"; "keep up the good work" [syn:
         prolong, sustain, keep up]
    2: undergo (as of injuries and illnesses); "She suffered a
       fracture in the accident"; "He had an insulin shock after
       eating three candy bars"; "She got a bruise on her leg"; "He
       got his arm broken in the scuffle" [syn: suffer, sustain,
       have, get]
    3: provide with nourishment; "We sustained ourselves on bread
       and water"; "This kind of food is not nourishing for young
       children" [syn: nourish, nurture, sustain]
    4: supply with necessities and support; "She alone sustained her
       family"; "The money will sustain our good cause"; "There's
       little to earn and many to keep" [syn: sustain, keep,
       maintain]
    5: be the physical support of; carry the weight of; "The beam
       holds up the roof"; "He supported me with one hand while I
       balanced on the beam"; "What's holding that mirror?" [syn:
       hold, support, sustain, hold up]
    6: admit as valid; "The court sustained the motion"
    7: establish or strengthen as with new evidence or facts; "his
       story confirmed my doubts"; "The evidence supports the
       defendant" [syn: confirm, corroborate, sustain,
       substantiate, support, affirm] [ant: contradict,
       negate]