The Feeding Nipple is an invention of the Covenant for feeding their Grunts. Feeding Nipples are located in many facilities where Grunts are kept, such as Methane Tanks, and Covenant starships.
Grunts, having such a low place in society, exist mainly to feed at these nipples as one of their few real pleasures. They contain a bubbling paste that the Grunts appear to love, but, according to Lieutenant McKay, smells like "old socks wrapped in rotting cheese."
They have been mentioned many, many times, but one has yet to be seen.