'de-anglicize' originates from a combination of the prefix 'de-' (from Latin 'de-' meaning 'away, off, reverse') and 'anglicize' (from French 'angliciser' ultimately from Latin 'Anglicus' meaning 'English')

'Anglicus' (Latin, meaning 'English') gave rise to Old French forms such as 'angliciser', which entered English as 'anglicize'; later the productive prefix 'de-' was attached to form 'de-anglicize', literally 'to reverse anglicize'

Initially 'anglicize' meant 'to make English in form or character'; with the prefix 'de-' the combined form came to mean 'to remove or reverse English characteristics', a straightforward negation/reversal of the original meaning