'postpygidial' originates from Latin prefix 'post-' (originating in Latin) meaning 'after' and from New Latin/Greek-derived 'pygidium' (from Greek 'pygidion') where 'pygidion' meant 'little rump/buttock'.

'pygidium' entered English via New Latin from Greek 'pygidion'; in scientific English the prefix 'post-' was later attached to form the compound adjective 'postpygidial' (also written 'post-pygidial') used in zoological descriptions.

Initially the elements meant 'after' + 'little rump', and the compound has retained the specialized anatomical meaning 'located behind the pygidium' in modern usage.