'immunitary' originates from Latin, specifically the word 'immunitas' (from 'immunis'), where 'immunis' meant 'exempt' or 'not subject to duty'.

'immunitas' passed into Medieval/Modern Latin and French as 'immunité' and English as 'immunity'; the adjective 'immunitary' was later formed in English from these roots to mean 'relating to immunity'.

Initially, the root referred to the state of being exempt (from public duties); over time it broadened to mean protection from disease or other forms of exemption, giving the modern sense 'relating to immunity'.