'polymorphonuclear-rich' originates from a modern English formation combining Greek and Latin elements: Greek 'poly-' (from Greek 'polús') meaning 'many', Greek 'morphē' meaning 'shape', Latin 'nucleus' meaning 'kernel/core', together with the English adjectival/combinative element '-rich' meaning 'having an abundance of'.

'polymorphonuclear' arose in medical English in the late 19th to early 20th century to describe leukocytes with multilobed nuclei (literally 'many-shaped nucleus'); the compound adjective 'polymorphonuclear-rich' is a later, transparent formation produced by attaching '-rich' to indicate abundance of those cells.

Initially the components literally described 'many-shaped nuclei'; over time the compound became a technical term for a class of white blood cells (chiefly neutrophils), and the derived phrase came to mean 'abundant in polymorphonuclear cells'.