n. An activist who promotes breast-feeding over the use of infant formula.
During the 1990s, Sears established himself as the most visible breast-feeding spokesman in the United States. … So loyal fans and followers of Sears were shocked to find that his site featured prominent banner ads promoting infant formula. …, which debuted in November, represents Sears' bid to expand his media presence to the Internet. The site has become the target of an angry letter-writing campaign by pro-breast-feeding parents, medical professionals and 'lactivists' all over the world.
—Katie Allison Granju, “Has 'America's pediatrician' sold out?,” Chicago Sun-Times, February 02, 2001
1999 (earliest)
Become a Lactivist!: The Very Best Resources for Learning More About Breastfeeding Advocacy
—Katie Allison Granju, Attachment Parenting, Atria, August 01, 1999
This word is a satisfying blend of the prefix lacto-, "milk" and activist. A synonym (seen almost exclusively on the Web) is nipple Nazi. Many thanks to subscriber Clint Stevens for letting me know about this term.