'immunostimulatory' originates from two elements: 'immuno-' and 'stimulatory'. 'Immuno-' ultimately comes from Latin 'immunis' meaning 'exempt', used in scientific contexts to mean 'relating to immunity'. 'Stimulatory' comes from Latin 'stimulus' meaning 'goad' or 'incitement', via 'stimulare' meaning 'to incite' and the adjectival suffix '-atory'.

'immune' entered English from Latin 'immunis' through Old French and Middle English; 'stimulus' gave rise to the verb 'stimulare' in Late Latin and then to English 'stimulate' (via Middle French/Latin-based scientific vocabulary). The compound form combining immuno- + stimulatory is a modern scientific formation used in biomedical English to describe substances or actions that stimulate immunity.

Initially 'immunis' meant 'exempt' in Latin; in modern scientific usage the prefix 'immuno-' has shifted to mean 'relating to the immune system'. 'Stimulatory' originally meant 'serving as a stimulus' and has retained that core meaning; together the compound now specifically means 'causing stimulation of the immune system.'