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]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "thwart":
   across, across the grain, annul, athwart, athwartships, baffle,
   balk, beat, bench, bendwise, bias, biased, biaswise, bilk, blast,
   block, brace, brave, bring to nothing, buffer, cancel, cancel out,
   cast down, catercorner, catercornered, challenge, check, checkmate,
   circumvent, come to nothing, confound, confront, contrariwise,
   contravene, contrawise, counter, counteract, counterbalance,
   countermand, counterwork, crisscross, cross, cross-grained,
   crossing, crossway, crossways, crosswise, curb, dash, defeat,
   defeat expectation, defy, destroy, diagonal, disappoint, discomfit,
   disconcert, discountenance, dish, disillusion, disrupt, dissatisfy,
   elude, flummox, foil, foul up, frustrate, gum up, hinder, impede,
   invalidate, kittycorner, knock the chocks, let down, negate,
   negativate, negative, neutralize, nonplus, nullify, oblique,
   obliquely, obstruct, offset, oppose, overthwart, perplex, queer,
   restrain, ruin, sabotage, scotch, short-circuit, sideways,
   sidewise, slant, spike, spoil, stonewall, stop, stultify, stump,
   style, stymie, tantalize, tease, thwartly, thwartways, transversal,
   transverse, transversely, traverse, undo, upset, vitiate, void