We go by "Baby Snacks" in our house (as opposed to "Noms" which means solid food). We started out with "Boob", as in "He wants a Boob" or "You, my child, are a Boob fiend".
The trouble with "Boob", however apt it is, is that eventually nursing babies turn into older nursing babies, even nursing toddlers. We let a "Boob" slip out every now and then (literally and figuratively!), but we try to stick to "Baby Snacks", in hopes that Rhys will pick up the term when he's old enough to ask to nurse. Imagine a toddler out and about for the day, clammoring for "Booooooobbbbbbb!!!!", and I think it is pretty easy to see the appeal of a code word.
A quick purusal of the internet shows code words such as: "Nursies", "Snuggle" and "Side" (as in "the other side").
Do you use a code word for breastfeeding? What is it?