'pro-opium' originates from the Modern English combination of the prefix 'pro-' (from Latin 'pro') meaning 'for' and the noun 'opium' (from Latin 'opium', itself from Greek 'opion').

'opium' comes to English via Latin 'opium' from Greek 'opion' (diminutive of 'opos' meaning 'juice'); the productive English prefix 'pro-' (from Latin 'pro', 'for') was attached in Modern English to form the compound 'pro-opium'.

Initially it would have the straightforward sense 'for opium' (i.e., in favor of opium); this literal sense remains the primary meaning in contemporary usage.