'injectable' is formed in modern English by adding the adjectival suffix '-able' to the verb 'inject'. 'Inject' originates from Latin, specifically the word 'injicere', where 'in-' meant 'into' and 'jacere' meant 'to throw'.

'inject' passed into English via Late Latin and Old French forms (e.g. Late Latin 'injectus', Old French 'injecter') and became Middle/Modern English 'inject'; from this the adjective 'injectable' was formed in English by affixation ('-able').

Originally, Latin 'injicere' meant 'to throw in' or 'throw into'; over time the sense narrowed to 'introduce (a substance) into a body or space', and 'injectable' now means 'able to be introduced by injection' or 'intended for injection'.