'non-isotonic' originates from the prefix 'non-' (Latin 'non', meaning 'not') combined with 'isotonic', which itself comes from Greek elements 'isos' meaning 'equal' and 'tonos' meaning 'tension' or 'pressure'.

'isotonic' was formed in modern scientific English from Greek 'isos' + 'tonos' to describe solutions of 'equal tension'; the productive English prefix 'non-' was later attached to create the negated form 'non-isotonic'.

Initially, 'isotonic' meant 'having equal tension/pressure'; the negated form 'non-isotonic' has consistently meant 'not isotonic' (i.e., unequal osmotic pressure) and has remained a straightforward negative formation without major semantic shift.