'unsympathetic' originates from Greek via Late Latin and French, specifically from Greek 'sympathetikos' (through Latin 'sympathia'/'sympatheticus'), where the prefix 'sym-' meant 'with' and the root 'path-' (from 'pathos') meant 'feeling'.

'unsympathetic' changed from Late Latin and French forms of 'sympathy' and 'sympathetic' (e.g. Latin 'sympathia', Medieval Latin 'sympatheticus'), entered Middle English as 'sympathetic', and later acquired the negative prefix 'un-' to form 'unsympathetic'; the adverbial form 'unsympathetically' developed by adding '-ally'.

Initially related to 'feeling with' or 'shared feeling' (mutual sympathy), the sense shifted to 'feeling pity or understanding', and then the prefixed form 'unsympathetic' came to mean 'not showing pity or understanding'; 'unsympathetically' now means 'in a manner showing no sympathy'.