'antineutron' originates from combining the Greek prefix 'anti-' meaning 'against' or 'opposite' and the modern English noun 'neutron,' which itself derives from 'neutral' with the particle suffix '-on'.

'neutron' developed from Latin 'neuter' ('neither'/'neutral') into English 'neutral' and later the particle name 'neutron'; 'anti-' comes from Greek 'antí' and was attached in modern scientific nomenclature to denote the antiparticle.

Initially it literally meant 'opposite of a neutron'; over time it came to be used specifically in particle physics to denote 'the neutron's antiparticle' with defined quantum properties.