Wiktionary (0.00 / 0 votes) Rate this definition: athwart (Adverb). From side to side; across. athwart (Adverb). Across the path (of something). athwart (Preposition). From one side to the other side of. The stars moved slowly athwart the sky. athwart (Preposition). Across the line of a ship's course or across its deck. The damaged mainmast fell athwart the deck, destroying the ship's boat. athwart (Preposition). Across the path or course of; opposing. Origin: From a- + thwart.