While “athwart” can be an adverb (modifying how a verb is used), it’s also used as a preposition. Prepositions indicate relationships between nouns and pronouns. Those relationships can pertain to location (“on,” “beside”), direction/movement (“into,” “toward”), time (“before,” “since”), cause (“because,” “for”), and manner (“with,” “by”). But these common prepositions aren’t the only options; plenty of other old-fashioned prepositions have fallen out of regular usage. “Athwart” is one such word. It means “from side to side of” — imagine sitting next to someone on a bench, but facing away from them. You would be sitting athwart (and that’s how the word is used as an adverb, as well).