'pro-polygamy' is formed by the English prefix 'pro' (from Latin 'pro' meaning 'for, in favor of') attached to 'polygamy' (from Greek 'polygamia', from 'poly-' meaning 'many' + 'gamos' meaning 'marriage').

'polygamy' originates from Greek 'polygamia' and entered English via Latin and French; the modern compound 'pro-polygamy' is an English formation using the productive prefix 'pro-' to denote support and thus arose in modern English usage.

Initially, 'pro-' simply indicated 'for' and 'polygamy' meant 'many marriages'; together in 'pro-polygamy' the meaning became the specific stance of being 'in favor of polygamy' (supportive or advocating its practice or legalization).