'assentaneous' originates from Latin, specifically from the verb 'assentire' (also seen as 'assentare'), where the prefix 'ad-' (seen in assimilated form 'as-') meant 'to/toward' and 'sentire' meant 'to feel' or 'to perceive'. The English adjective was formed by combining 'assent-' with the adjectival suffix '-aneous'.

'assentaneous' developed via Medieval Latin as 'assentaneus' (or similar forms) and entered English usage as the adjective 'assentaneous', built on the noun/verb 'assent' (from Latin 'assensus'/'assentire').

Initially related to the act of giving assent or agreement ('to feel toward' in a figurative sense), it has retained that core sense and is used to describe someone or something that expresses or exhibits agreement.