'argillous' originates from Latin, specifically the word 'argilla', where 'argilla' meant 'clay'.

'argillous' was formed in English from Latin 'argilla' with the adjectival suffix '-ous' (via Neo-Latin/Medieval Latin formations such as 'argillosus'), eventually becoming the modern English adjective 'argillous'.

Initially it meant 'of or relating to clay,' and this basic meaning has remained largely the same in modern usage as 'consisting of or containing clay'.