'proslavery' originates from Modern English, specifically formed from the prefix 'pro-' (from Latin 'pro' meaning 'for, in favor of') combined with 'slavery' (the noun denoting the condition or practice of slavery).

'Slavery' traces back through Middle English and Old French (e.g. Old French 'esclavage') to Medieval Latin 'sclavus' (meaning 'Slav', a people from whom many were taken as slaves); 'slave' entered English by way of these forms, and 'slavery' developed as the noun. The compound 'proslavery' was formed in English (chiefly 19th century) by prefixing 'pro-' to 'slavery' to denote support for the institution.

The compound originally and consistently meant 'in favor of slavery' (i.e., supporting the institution); its basic meaning has remained stable, though historical usage is tied to specific political contexts (notably 18th–19th century debates).