'baptise' originates from Greek via Late Latin and Old French. Specifically, it comes from Late Latin 'baptizare', from Greek 'baptizein' (βαπτίζειν), where the root 'bapto/bapt-' meant 'to dip' or 'to immerse'.

'baptizein' (Greek) passed into Late Latin as 'baptizare', then through Old French ('baptiser') and Middle English ('baptisen'), eventually becoming the modern English 'baptise'.

Initially it meant 'to dip or immerse'; over time it came to mean specifically 'to administer the Christian rite of baptism' (and by extension to name or consecrate in such a rite).