'assentatorily' originates from Latin, specifically the word 'assentire', where the prefix 'ad-' (later assimilated to 'as-') meant 'to/toward' and the root 'sentire' meant 'to feel' or 'to think'. The English form developed by combining the adjective-forming suffix '-atory' and the adverbial suffix '-ly'.

'assentatorily' changed from Late Latin/Medieval Latin 'assentari' and the adjective form 'assentatorius' into Middle English forms (e.g. 'assentatory') and later gained the adverbial ending to become modern 'assentatorily'.

Initially connected to the Latin sense 'to feel or be of one mind' (i.e. to agree), it evolved into the modern adverbial sense 'in a manner expressing agreement'.