Athwart is an adjective or adverb meaning toward one side of the boat or another, usually perpendicular to the fore-and-aft center line of the boat. The word refers to a thwart which is a horizontal structure on a boat that serves as a seat.
Athwart is similar to abeam, except that athwart usually refers to things that are inside the boat, and abeam refers to things outside the boat. Abeam also specifically refers to something that is at a 90 degree angle to the boat.
The helmsman and the navigator should stand athwart of each other.
Wait until the buoy is abeam of us before you tack.