'aplacophorous' originates from New Latin scientific formation 'Aplacophora', ultimately from Greek elements where 'a-' meant 'not', 'plax/plac-' (from plakos) meant 'plate', and 'phoros' meant 'bearing'.

'aplacophorous' was formed in modern scientific English from New Latin 'Aplacophora' (the name for the group of unplated molluscs) and adapted with the adjectival suffix '-ous' (from Greek/Latin formations) to produce the modern English adjective 'aplacophorous'.

Initially it meant 'not bearing plates' specifically as a descriptive term for certain molluscs; over time it has been used more generally in zoological contexts to mean 'relating to or characteristic of Aplacophora' or 'lacking plates/shells'.