'arsenous' originates from New Latin 'arsenous', formed from 'arsenic' + the adjectival suffix '-ous'.

'arsenous' derives ultimately from Medieval/Modern Latin 'arsenicum' (from Greek 'arsenikon' ), where Greek 'arsenikon' () passed into Medieval Latin as 'arsenicum' and later into English as 'arsenic', with the adjective 'arsenous' formed in New/Modern Latin and adopted into English.

Initially the root referred to the substance named for its 'potency' in Greek usage; over time the English adjective came to mean specifically 'containing or relating to arsenic', and in chemistry it now commonly denotes arsenic in the trivalent (III) oxidation state.