The Collaborative International Dictionary of English v.0.48:

Ruin \Ru"in\, n. [OE. ruine, F. ruine, fr. L. ruina, fr. ruere,
   rutum, to fall with violence, to rush or tumble down.]
   1. The act of falling or tumbling down; fall. [Obs.] "His
      ruin startled the other steeds." --Chapman.
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   2. Such a change of anything as destroys it, or entirely
      defeats its object, or unfits it for use; destruction;
      overthrow; as, the ruin of a ship or an army; the ruin of
      a constitution or a government; the ruin of health or
      hopes. "Ruin seize thee, ruthless king!" --Gray.
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   3. That which is fallen down and become worthless from injury
      or decay; as, his mind is a ruin; especially, in the
      plural, the remains of a destroyed, dilapidated, or
      desolate house, fortress, city, or the like.
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            The Veian and the Gabian towers shall fall,
            And one promiscuous ruin cover all;
            Nor, after length of years, a stone betray
            The place where once the very ruins lay. --Addison.
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            The labor of a day will not build up a virtuous
            habit on the ruins of an old and vicious character.
                                                  --Buckminster.
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   4. The state of being dcayed, or of having become ruined or
      worthless; as, to be in ruins; to go to ruin.
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   5. That which promotes injury, decay, or destruction.
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            The errors of young men are the ruin of business.
                                                  --Bacon.
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   Syn: Destruction; downfall; perdition; fall; overthrow;
        subversion; defeat; bane; pest; mischief.
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The Collaborative International Dictionary of English v.0.48:

Ruin \Ru"in\, v. i.
   To fall to ruins; to go to ruin; to become decayed or
   dilapidated; to perish. [R.]
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         Though he his house of polished marble build,
         Yet shall it ruin like the moth's frail cell. --Sandys.
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         If we are idle, and disturb the industrious in their
         business, we shall ruin the faster.      --Locke.
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The Collaborative International Dictionary of English v.0.48:

Ruin \Ru"in\, v. t. [imp. & p. p. Ruined;p. pr. & vb. n.
   Ruining.] [Cf. F. ruiner, LL. ruinare. See Ruin, n.]
   To bring to ruin; to cause to fall to pieces and decay; to
   make to perish; to bring to destruction; to bring to poverty
   or bankruptcy; to impair seriously; to damage essentially; to
   overthrow.
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         this mortal house I'll ruin.             --Shak.
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         By thee raised, I ruin all my foes.      --Milton.
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         The eyes of other people are the eyes that ruin us.
                                                  --Franklin.
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         By the fireside there are old men seated,
         Seeling ruined cities in the ashes.      --Longfellow.
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WordNet (r) 3.0 (2006):

ruin
    n 1: an irrecoverable state of devastation and destruction; "you
         have brought ruin on this entire family" [syn: ruin,
         ruination]
    2: a ruined building; "they explored several Roman ruins"
    3: the process of becoming dilapidated [syn: dilapidation,
       ruin]
    4: an event that results in destruction [syn: ruin,
       ruination]
    5: failure that results in a loss of position or reputation
       [syn: downfall, ruin, ruination]
    6: destruction achieved by causing something to be wrecked or
       ruined [syn: laying waste, ruin, ruining, ruination,
       wrecking]
    v 1: destroy completely; damage irreparably; "You have ruined my
         car by pouring sugar in the tank!"; "The tears ruined her
         make-up" [syn: destroy, ruin]
    2: destroy or cause to fail; "This behavior will ruin your
       chances of winning the election"
    3: reduce to bankruptcy; "My daughter's fancy wedding is going
       to break me!"; "The slump in the financial markets smashed
       him" [syn: bankrupt, ruin, break, smash]
    4: reduce to ruins; "The country lay ruined after the war"
    5: deprive of virginity; "This dirty old man deflowered several
       young girls in the village" [syn: deflower, ruin]
    6: fall into ruin

The Devil's Dictionary (1881-1906):

RUIN, v.  To destroy.  Specifically, to destroy a maid's belief in the
virtue of maids.