'postmarginal' originates from Latin and Medieval Latin elements: the prefix 'post-' (from Latin post) meaning 'after' combined with 'marginal' (from Medieval Latin marginalis), where the root 'margo/marginis' meant 'edge' or 'border'.

'postmarginal' formed in English by combining the Latin-derived prefix 'post-' with the adjective 'marginal' (itself from Old French/Medieval Latin and ultimately Latin 'margo, marginis'), resulting in a compound used in general and later specialized technical senses (e.g., entomology).

Initially it literally meant 'after the margin' or 'located beyond the edge'; over time it kept that core sense but developed a specialized anatomical/entomological meaning referring specifically to regions or veins just beyond an insect wing's margin.