'argillaceous' originates from Latin, specifically the word 'argillaceus', where 'argilla' meant 'clay' and the suffix '-aceous' meant 'of the nature of'.

'argillaceous' changed from Latin 'argillaceus' (used in Neo-Latin/Scientific Latin) and entered English usage as the modern adjective 'argillaceous'.

Initially, it meant 'of or pertaining to clay', and over time it has remained largely the same, now commonly used to mean 'clayey' in geological contexts.