Merriam-Webster's Collegiate® Dictionary, 11th Edition
THWART
I. transitive verb
Etymology: Middle English thwerten, from thwert, adverb
Date: 13th century
1. to run counter to so as to effectively oppose or baffle ; contravene , to oppose successfully ; defeat the hopes or aspirations of, to pass through or across, see: frustrate thwarter noun
II. adverb
Etymology: Middle English thwert, from Old Norse thvert, from neuter of thverr transverse, oblique; akin to Old High German dwerah transverse, oblique
Date: 14th century
athwart ,
III. adjective
Date: 14th century
situated or placed across something else ; transverse , thwartly adverb
IV. noun
Etymology: alteration of obsolete thought, thoft, from Middle English thoft, from Old English thofte; akin to Old High German dofta rower's seat
Date: circa 1736
a seat extending athwart a boat