'unceremonious' originates from Modern English, specifically formed by the negative prefix 'un-' + the adjective 'ceremonious', where 'un-' meant 'not' and 'ceremonious' is ultimately from Latin 'ceremonia' meaning 'a religious rite' or 'formal observance'.

'ceremonious' comes from Latin 'ceremonia' via Old French/Latin influence into Middle English; the adjective 'unceremonious' was formed in English by adding the negative 'un-' to 'ceremonious' in the Early Modern English period and developed into the modern word 'unceremonious'.

Initially formed to mean 'not ceremonious' (i.e., lacking ceremony or formality); over time it acquired senses emphasizing abruptness or lack of courtesy, giving the modern meanings 'informal/abrupt' and 'discourteous'.