'pro-disestablishment' is formed from the prefix 'pro-' (from Latin 'pro' meaning 'for') plus 'disestablishment' (built from the prefix 'dis-' from Latin 'dis-' meaning 'apart, away' and 'establishment' from Old French 'establissement' and Latin roots meaning 'to make firm'), where the whole term means 'for the removal of an established (church)'.

'disestablishment' developed from Middle English (and Old French) forms of 'establishment' with the negative prefix 'dis-'; political usage of 'pro-' attached to such nouns to indicate supporters (yielding 'pro-disestablishment') arose in modern political discourse about church-state relations.

Originally 'establish' referred to making something firm or set up; 'disestablishment' came to mean the removal of an established institution (especially a state church). 'Pro-disestablishment' has retained the sense 'in favor of that removal.'