'athwartships' originates from Middle English, specifically built from 'a-' + 'thwart', where 'thwart' (from Old Norse þvert/Old English þrēorþ/*thweorþ) meant 'across'.

'thwart' (from Old Norse þvert and related Old English forms) appeared in Middle English as 'thwart'/'athwart', and was later combined with 'ships' to form the nautical adverb 'athwartships' (across a ship).

Initially it meant simply 'across' or 'crosswise'; over time it specialized in nautical usage to mean 'across a ship' or 'sideways on a vessel', which is its primary modern sense.