'athwart' originates from the combination of the prefix 'a-' (from Old English 'on' or 'ā', meaning 'on' or 'in') and 'thwart', which goes back to Old Norse 'þverr' meaning 'across'.

'athwart' appeared in Middle English as a- + 'thwart' (Old English/Old Norse roots). 'Thwart' itself comes from Old Norse/Old English words for 'across' (e.g. Old Norse 'þverr', Old English 'þweorh'), and the prefixed form 'athwart' developed in Middle English into the modern form.

Initially it meant 'across' or 'crosswise'; over time it retained that spatial sense but also acquired figurative senses of being 'opposed' or 'in opposition'.