The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, v. t. [imp. & p. p. Thwarted; p. pr. & vb. n.
   Thwarting.]
   1. To move across or counter to; to cross; as, an arrow
      thwarts the air. [Obs.]
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            Swift as a shooting star
            In autumn thwarts the night.          --Milton.
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   2. To cross, as a purpose; to oppose; to run counter to; to
      contravene; hence, to frustrate or defeat.
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            If crooked fortune had not thwarted me. --Shak.
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            The proposals of the one never thwarted the
            inclinations of the other.            --South.
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The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, v. i.
   1. To move or go in an oblique or crosswise manner. [R.]
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   2. Hence, to be in opposition; to clash. [R.]
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            Any proposition . . . that shall at all thwart with
            internal oracles.                     --Locke.
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The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, a. [OE. [thorn]wart, [thorn]wert, a. and adv.,
   Icel. [thorn]vert, neut. of [thorn]verr athwart, transverse,
   across; akin to AS. [thorn]weorh perverse, transverse, cross,
   D. dwars, OHG. dwerah, twerh, G. zwerch, quer, Dan. & Sw.
   tver athwart, transverse, Sw. tv[aum]r cross, unfriendly,
   Goth. [thorn]wa['i]rhs angry. Cf. Queer.]
   1. Situated or placed across something else; transverse;
      oblique.
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            Moved contrary with thwart obliquities. --Milton.
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   2. Fig.: Perverse; crossgrained. [Obs.] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, adv. [See Thwart, a.]
   Thwartly; obliquely; transversely; athwart. [Obs.] --Milton.
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The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, prep.
   Across; athwart. --Spenser.
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   Thwart ships. See Athwart ships, under Athwart.
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The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, n. (Naut.)
   A seat in an open boat reaching from one side to the other,
   or athwart the boat.
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WordNet (r) 3.0 (2006):

thwart
    n 1: a crosspiece spreading the gunnels of a boat; used as a
         seat in a rowboat [syn: thwart, cross thwart]
    v 1: hinder or prevent (the efforts, plans, or desires) of;
         "What ultimately frustrated every challenger was Ruth's
         amazing September surge"; "foil your opponent" [syn:
         thwart, queer, spoil, scotch, foil, cross,
         frustrate, baffle, bilk]