What do you call a nurse with dirty knees?

The joke “What do you call a nurse with dirty knees? The ‘head’ nurse” is a play on words that relies on a double entendre. In a medical context, a “head nurse” is a senior nurse who oversees other nurses and is responsible for the management of care in a unit or department. However, the joke introduces the idea of “dirty knees,” which is often associated with kneeling, and then ties it to the term “head” nurse suggesting that the nurse got her knees dirty through oral sex activity. The humor arises from the juxtaposition of the professional title “head nurse” with the suggestive image evoked by “dirty knees.”