'defogging' is formed from the prefix 'de-' (from Latin 'dē' meaning 'away, remove') plus the English noun 'fog' (from Middle English 'fogge', of uncertain origin).

'defog' was created in modern English by prefixing Latin-derived 'de-' to the existing word 'fog' (Middle English 'fogge'); adding the gerundial suffix '-ing' produced 'defogging' as the noun/participle.

Initially it meant 'to remove fog or condensation' (a literal sense). Over time it has also been used figuratively (e.g., 'to defog one's thinking') and adjectivally for devices/features that prevent or clear fog.