'post-pygidial' is formed in modern English from the prefix 'post-' (from Latin 'post', meaning 'after') combined with 'pygidial', which derives from New Latin 'pygidium' (from Greek 'pygidion', diminutive of 'pygē' meaning 'rump').

'pygidial' comes from New Latin zoological usage of 'pygidium' (from Greek 'pygidion'), and the compound 'post-pygidial' was later created in technical English by adding the prefix 'post-' to indicate position 'after' the pygidium.

Originally the roots meant 'after' (post-) and 'rump' (pygidion); in technical biological usage this evolved into the specific locational sense 'situated behind the pygidium'.