'argillitic' originates from Latin, specifically the word 'argilla', where 'argilla' meant 'clay'; the adjectival suffix '-itic' (from Greek '-itikos') was added to form a term meaning 'of or relating to clay.'

'argillitic' developed through Late Latin and Romance forms: Latin 'argilla' > Old French 'argile' (and Medieval Latin forms such as 'argillaceus') > English formations like 'argillaceous' and the noun 'argillite', eventually giving the modern English adjective 'argillitic'.

Initially it referred to 'clay' (the material) via Latin 'argilla', but over time it evolved into an adjectival sense meaning 'composed of or relating to clay' (i.e., 'clay-like' or 'clay-containing').