'pro-ejaculatory' originates from Latin, specifically the prefix 'pro-' and the adjective 'ejaculatory', where 'pro-' meant 'for' or 'forward' and 'ejaculatory' derives from Latin 'eiaculari' (or 'ejaculari') meaning 'to throw out'.

'ejaculatory' changed from Latin 'eiaculari/eiaculatio' (to throw out; a throwing forth) through Late Latin and Middle English into the English verb 'ejaculate' and the adjective 'ejaculatory'; the modern compound 'pro-ejaculatory' is formed by combining the prefix 'pro-' with 'ejaculatory'.

Initially it referred generally to 'throwing out' or expelling; over time the root specialized to mean the expulsion of semen (ejaculation), and the adjective evolved to describe things relating to or promoting that physiological process; adding 'pro-' gives the sense 'in favor of' or 'promoting' ejaculation.