'anisotonic' originates from Greek-derived elements, specifically the prefix 'an-' (from Greek 'an-') meaning 'not' combined with 'isotonic' (from Greek 'isos' meaning 'equal' and 'tonos' meaning 'tension').

'anisotonic' arose as a scientific formation by attaching 'an-' to 'isotonic' (it was formed in New Latin/scientific usage) and entered English in modern scientific contexts as 'anisotonic'.

Initially it meant 'not equal in tension' in a general sense; over time the term has been used more specifically for 'having unequal osmotic pressure' in physiological and chemical contexts.