From side to side; across.

"Above, the stars appeared to move slowly athwart."

Across the path (of something).

"a fleet standing athwart our course"

Wrongly; perplexingly.

From one side to the other side of.

"The stars moved slowly athwart the sky."

Across the line of a ship's course or across its deck.

"The damaged mainmast fell athwart the deck, destroying the ship's boat."

Across the path or course of; opposing.