'bacteriaceous' originates from New Latin/Modern formation, specifically from 'bacterium' (itself from Greek 'bakterion') combined with the Latin-derived adjectival suffix '-aceous' meaning 'of or pertaining to'.

'bacterium' comes from Greek 'bakterion' (a diminutive of 'baktron', meaning 'rod' or 'stick') which was Latinized in New Latin as 'bacterium'; the suffix '-aceous' derives from Latin '-aceus' (via Medieval/Modern Latin use) meaning 'of or pertaining to'; the combination produced the English adjective 'bacteriaceous'.

Initially it meant 'pertaining to or resembling bacteria', and over time this core meaning has largely remained the same in modern usage.