'barbarise' originates from Late Latin and Medieval Latin usage, specifically from the Late Latin/Medieval Latin verb 'barbarizare', ultimately from Greek 'barbarizein', where the root 'barbar-' meant 'foreign, strange' (imitative of unintelligible speech).

'barbarizein' (Greek) passed into Late Latin as 'barbarizare', and via Old French (and Middle English influence) evolved into Middle English forms such as 'barbarisen', eventually becoming modern English 'barbarise' (British) / 'barbarize' (American).

Initially it meant 'to treat as foreign or to make foreign/uncivil' (root sense tied to 'barbarian' as 'one who speaks unintelligibly'), and over time this developed into the current senses of 'make barbarous; degrade or vulgarise'.