'ducal' originates from Latin, specifically the word 'ducalis', where the root 'duc-' (from 'dux') meant 'leader' or 'commander' (later 'duke').

'ducal' passed from Latin 'ducalis' into Medieval/Old French forms and then into Middle English as 'ducal', eventually becoming the modern English adjective 'ducal'.

Initially it referred to matters relating to a 'leader' or 'commander' (dux), and over time the meaning narrowed to matters specifically relating to a 'duke' or his duchy; the core sense (relation to a duke) has been retained.